ways to keep a high level captive

[King of Nowhere]

we are fighting a cabal of... not exactly druid liches, but they are a homebrew that's similar to liches. most relevant, they have the usual undead immunities, and they can shapeshift. including elemental form. and they have a phylactery.
they are no match for us in direct combat, but they keep coming back, and so far we've been unable to track their phylacteries or find some permanent ways to get rid of them.

i was considering whether we could just capture them and keep them locked up to prevent them from pestering us.
capturing some alive is trivial enough, we are high level.
keeping them locked up is another thing entirely.
A permanet antimagic field would do the trick, but we don't have one available. those druids have all their spellcasting capacities, but those would not be a huge deal. shapeshift is the major problem.
turning into a earth elemental allows to pass through stone, and gives enough strenght to break most cages. fire elemental can destroy mamny of the materials used to imprison them. It's not clear how much an air elemental can squeeze through a small hole, but it's certainly another avenue of escape. also, shapeshifting is a good way to get rid of any bondage or manacles (i guess wild clasping manacles would be possible, but who makes them?), which would have been a good way to stop them from casting.
Keeping them unconscious or drugged is impossible, as they are undead.
not only that, but we'd even have to stop them from suiciding, because if they do, they just regenerate with their phylactery.

Is there a reasonably practical way we could keep some of those guys locked up? party has access to 9th level magic, though only a handful of arcane spells are available so far

[the_tick_rules]

{Scrubbed}

antimagic shackles - expensive but they would do the trick.

[Goaty14]

A permanet antimagic field would do the trick, but we don't have one available. those druids have all their spellcasting capacities, but those would not be a huge deal. shapeshift is the major problem.

Well, an Antimagic Field (https://www.d20srd.org/srd/spells/antimagicField.htm) blocks most spells, SLAs, and Supernatural abilities, which includes wild shape/elemental form abilities.

Grabbing a pair of Antimagic Shackles (Book of Exalted Deeds, 132,000 GP) could easily do the deed, but I suspect that the cost of doing that to successfully entrap one of these druids is much more costly compared to smacking them down every once in a while.

Speaking of Wildling Clasps, it's notable that you don't have to custom-make them for certain items. Rather, you just drop 4k to buy it, and then attach it to any item that can be worn on the body (such as manacles). You might be concerned with if the druid could take the clasps off while wearing them, which should be up to your DM.

I don't know how many teleportation options druids have, but if you double check that they have none, you could permanency a cube made out of walls of force (it doesn't look like Forcecage is able to be made permanent, sadly), make sure it's air-proof, and just Teleport the druids in there during battle.
Edit: I checked; druids do get teleportation options at higher levels. After stripping them of plants/dirt before sticking them in there (given Transport Via Plants, and Master Earth), Shuffle (Shining South, 6th level, obscure?), and Unicorn Heart (CM, 7th) both allow for short-range teleportation and aren't dependent on plants/earth/storms like their other options are.

Plus, just realized that sticking more than 1 druid in the same place means they could punch each other to death and therefore escape anyways... :p

[Biffoniacus_Furiou]

Spells like Imprisonment, Temporal Stasis, or similar should work. Anything that traps and incapacitates them without destroying or killing them.

[AvatarVecna]

Spells like Imprisonment, Temporal Stasis, or similar should work. Anything that traps and incapacitates them without destroying or killing them.

Came here to say, stuff like this is literally what Imprisonment is for.

Alternatively, you could make something convoluted with like...Genesis up a demiplane with Dead Magic and Fast Time. Like, instead of "super-fast inside normal outside" like people use to break artificer, you make the reverse, where every second inside the plane is a thousand years in the material. So like if you push them in and close the gate, then 500 years later you finally are ready to pull them out, from their perspective it's been half a second since they got pushed in and they haven't even had time to attempt to escape. I'm sure something less severe than that would suffice too. Honestly, even a day per second would be sufficient to keep from giving them enough time to figure out a spell or form that's capable of working on a dead magic plane to get them out of there.

Oh you could probably abuse a pool of quintessence for something like this. Among other things.

[Angrith]

Ability score damage might be viable. An item of use-activated touch of idiocy should only cost 18,000 if I've done my math right. Just tie 'em up and poke them a bunch. They can't cast if they're wisdom is 1. You'd have to repeat every 30 minutes though. You could use other ability damage but risk accidentally dropping them below 0.

EDIT: Forgot they were druids. This only solves half the problem. And they're pseudo-liches so not a living creature, nevermind.

[Telonius]

You don't need an AMF. All you need is a Breastplate, some sovereign glue, and a way to stuff the Druid into it.


A druid who wears prohibited armor or carries a prohibited shield is unable to cast druid spells or use any of her supernatural or spell-like class abilities while doing so and for 24 hours thereafter.

[Goaty14]

Ability score damage might be viable. An item of use-activated touch of idiocy should only cost 18,000 if I've done my math right. Just tie 'em up and poke them a bunch. They can't cast if they're wisdom is 1. You'd have to repeat every 30 minutes though. You could use other ability damage but risk accidentally dropping them below 0.

EDIT: Forgot they were druids. This only solves half the problem. And they're pseudo-liches so not a living creature, nevermind.

I think you're on to something though. Despite being immune to poison, ability drain, and ability damage/drain to their physical attributes, you could still damage their mental ability scores, and turn them into vegetables if you keep applying the means of lowering their stats? According to here (http://archive.wizards.com/default.asp?x=dnd/glossary&term=Glossary_dnd_abilityscoreloss&alpha=), you can't reduce a stat below 0, and reducing anything that's not CON to 0 doesn't have a negative effect beyond immobilizing them. That said, however, the moment they have a 1 in a stat, they could shapeshift and escape, meaning you'd need a way to give them ability damage within 6 seconds (1 round) of them waking up, like, a mindless minion with an eternal readied action to smack them with ability damage, or a spell clock that triggers in a 24 hour loop starting at the moment you first put them to 0, or some other method I'm not aware of

[Biffoniacus_Furiou]

You don't need an AMF. All you need is a Breastplate, some sovereign glue, and a way to stuff the Druid into it.


A druid who wears prohibited armor or carries a prohibited shield is unable to cast druid spells or use any of her supernatural or spell-like class abilities while doing so and for 24 hours thereafter.

Slight of Hand a tiny steel buckler into their pocket!

[Angrith]

I think you're on to something though. Despite being immune to poison, ability drain, and ability damage/drain to their physical attributes, you could still damage their mental ability scores, and turn them into vegetables if you keep applying the means of lowering their stats? According to here (http://archive.wizards.com/default.asp?x=dnd/glossary&term=Glossary_dnd_abilityscoreloss&alpha=), you can't reduce a stat below 0, and reducing anything that's not CON to 0 doesn't have a negative effect beyond immobilizing them. That said, however, the moment they have a 1 in a stat, they could shapeshift and escape, meaning you'd need a way to give them ability damage within 6 seconds (1 round) of them waking up, like, a mindless minion with an eternal readied action to smack them with ability damage, or a spell clock that triggers in a 24 hour loop starting at the moment you first put them to 0, or some other method I'm not aware of

Is there a way to embed something that does continuous damage and apply it as a rider? Then you could use one of many ways to heal hp to keep them (un)living while their stats are continuously drained. I am not familiar enough with 3.5 to know the answer to this one.

[FauxKnee]

Spells like Imprisonment, Temporal Stasis, or similar should work. Anything that traps and incapacitates them without destroying or killing them.

The spell amber sarcophagus from the Book of Exalted Deeds is another good option. It's much cheaper than Temporal Stasis, a lower spell level, a ranged touch attack instead of a melee touch attack, and offers no save. The tradeoff is that it has a limited duration and is easier to end than the other two options. Given that amber is hardened plant matter, it arguably counts as "something that was once alive" and is therefore eligible for unguent of timelessness. Furthermore, you can presumably move the captive inside their amber cocoon. You put it somewhere more secure, or just somewhere out of the way. If you wanted to push them through a gate, you could send them to a plane that's timeless with respect to magic. Depending on their spell mechanic, you might also be able to just leave them adrift somewhere in outer space, floating in the void between stars.

You could also use polymorph any object, shifting them into a form where they can't cause trouble. You should be able to turn a human lich into a donkey permanently: +5 kingdom (animal), +2 (same size), +2 (same or lower intelligence.) Perhaps more amusing would be to turn them into a zombie. Creatures of the undead type can be polymorphed into other undead, and polymorph any object explicitly changes their intelligence score. You'd be able to make them mindless, incapable of taking any action of their own volition.

[SirNibbles]

Spark of Life followed by any type of mental ability drain to 0 will keep it knocked out.

[PoeticallyPsyco]

You don't need an AMF. All you need is a Breastplate, some sovereign glue, and a way to stuff the Druid into it.

This. Without spells, wildshape, or any other class features, they're going to be in a world of hurt. At this point simple chains will probably be enough, unless they're getting some serious perks from whatever the source of their undeath is.

One thing you probably want to watch out for is an uncaptured member of the group performing a jailbreak. Or any minions they have or can acquire.

[Bohandas]

Petrification springs to mind.

Another possibility is bodily dismembermemt, a la the Evil Dead series. Wielding a weapon or casting most spells is going to require them to have limbs. Without their limbs they're powerless (unless they have the still spell feat, in which case they still lose their highest level of spells)

Also, preventing then from ever having 8 uninterrupted hours of restful calm will prevent them from regaining spells

[Quertus]

My usual ways of dealing with similar problems include Trap the Soul, Imprisonment, Temporal Stasis, Flesh to Stone, Flesh to Glass, Mindrape, Exalted Mindrape, Diplomacy (muggle Mindrape), Gate (you wouldn't believe how many special snowflake worlds I've seen - including ones where the "suicide" option would result in the world eating the Druid's soul rather than them returning to those phylactery), Bluff (see previous parenthetical), Polymorph any Object, dismemberment, ability damage / ability drain / curses, antimagic / dead zones, and, on at least one occasion, crafting them into a sentient magical item.

In this particular case, I might consider using turning shenanigans, and just Rebuke them all.

Kudos to AvatarVecna for using quintessence, and to Telonius for the armor idea! I'll have to add those to my toolkit…

[King of Nowhere]

I think you're on to something though. Despite being immune to poison, ability drain, and ability damage/drain to their physical attributes, you could still damage their mental ability scores, and turn them into vegetables if you keep applying the means of lowering their stats?
good idea. how could we do that? the main avenue for ability damage is poison, which won't work in this case.




You could also use polymorph any object, shifting them into a form where they can't cause trouble. You should be able to turn a human lich into a donkey permanently: +5 kingdom (animal), +2 (same size), +2 (same or lower intelligence.) Perhaps more amusing would be to turn them into a zombie. Creatures of the undead type can be polymorphed into other undead, and polymorph any object explicitly changes their intelligence score. You'd be able to make them mindless, incapable of taking any action of their own volition.


Petrification springs to mind.

Liches are immune to polymorph, which I assume would also include petrification.
but those are not exactly liches, so it may be worth a try


You don't need an AMF. All you need is a Breastplate, some sovereign glue, and a way to stuff the Druid into it.
this depends on RAI. A druid is bound by his oaths to not use metal, but if someone forcibly stuffed him into metal gear and he's trying to get out, he's not violating anything. so, it may be ineffective.
on the other hand, perhaps metal specifically interferes with druid abilities, so it acts as a sort of lesser antimagic for them.

worth checking. we have a druid in the party, so his character should know for certain whether this would work.

[ixrisor]

These creatures presumably don’t have a constitution score and are therefore immune to ability damage and drain.

[SirNibbles]

These creatures presumably don’t have a constitution score and are therefore immune to ability damage and drain.

They're mentioned by the OP as having undead immunities. You can overcome those with Spark of Life (Spell Compendium, page 196):

"For the duration of the spell, the undead creature is subject to extra damage from critical hits (and thus sneak attacks), nonlethal damage, ability drain, energy drain, fatigue, exhaustion, and damage to its physical ability scores (though it still lacks a Constitution score and thus can't take Constitution damage) as if it were alive.

It loses its immunity to effects that require a Fortitude save, as well as its invulnerability to poison, sleep effects, paralysis, stunning, disease, and death effects."

And once you take damage, becoming immune to that type of damage doesn't magically heal you any more than becoming immune to fire after being damaged by a fireball.

[Biffoniacus_Furiou]

These creatures presumably don’t have a constitution score and are therefore immune to ability damage and drain.

Undead are only immune to ability damage to their physical ability scores. A Psychic Rogue or Psychic Assassin (http://archive.wizards.com/default.asp?x=dnd/psm/20040723a) with Mind Cripple and the Death's Ruin ACF in CC can absolutely disable them with Int damage.

[SirNibbles]

Undead are only immune to ability damage to their physical ability scores. A Psychic Rogue or Psychic Assassin (http://archive.wizards.com/default.asp?x=dnd/psm/20040723a) with Mind Cripple and the Death's Ruin ACF in CC can absolutely disable them with Int damage.

There's nothing in the undead type that contradicts the general rule that no Con score means you're immune to ALL ability damage:




A creature that has no Constitution has no body or no metabolism. It’s immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks.

Rules Compendium, page 105

[King of Nowhere]

actually, i don't know their full stats, but i think they do have a CON score. it's like they are only half undead.
still, negative energy heals them, they are immune to stunning and criticals, and they seem to have all the immunities that we have tested.

anyway, whether they would be immune to stat damage will not depend on this. when we have a dubious case, we don't dig through the rules too much; we present arguments and the DM makes a call, and the call then stays for the rest of the campaign.

[Biffoniacus_Furiou]

There's nothing in the undead type that contradicts the general rule that no Con score means you're immune to ALL ability damage:




A creature that has no Constitution has no body or no metabolism. It’s immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks.

Rules Compendium, page 105


The Undead Type (https://www.d20srd.org/srd/typesSubtypes.htm#undeadType) says otherwise, and specific trumps general:
"Not subject to critical hits, nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Strength, Dexterity, and Constitution), as well as to fatigue and exhaustion effects."

[SirNibbles]

The Undead Type (https://www.d20srd.org/srd/typesSubtypes.htm#undeadType) says otherwise, and specific trumps general:
"Not subject to critical hits, nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Strength, Dexterity, and Constitution), as well as to fatigue and exhaustion effects."




Traits: An undead creature possesses the following traits (unless otherwise noted in a creature’s entry).
—No Constitution score.
—Not subject to critical hits, nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Strength, Dexterity, and Constitution), as well as to fatigue and exhaustion effects.

Monster Manual, page 317


Specific here does not contradict general. It says they have no Con score and thus are immune to all ability damage. It then goes on to say they are immune to damage to their physical ability scores. It does not say they are vulnerable to ability damage to their mental ability scores. Since there is no contradiction to the general rule, it applies.

[Heliomance]

Dunking them in quintessence works quite nicely if you can get enough

[Fouredged Sword]

Traits: An undead creature possesses the following traits (unless otherwise noted in a creature’s entry).
—No Constitution score.
—Not subject to critical hits, nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Strength, Dexterity, and Constitution), as well as to fatigue and exhaustion effects.

Monster Manual, page 317


Specific here does not contradict general. It says they have no Con score and thus are immune to all ability damage. It then goes on to say they are immune to damage to their physical ability scores. It does not say they are vulnerable to ability damage to their mental ability scores. Since there is no contradiction to the general rule, it applies.

And further the general rule is overruled by the specific text of the spark of life spell. It doesn't matter why the undead is immune to ability damage. The spell spark of life explicitly undoes that immunity regardless of the immunity arising from the Undead type or a lack of a con score. It is clear that the intent of spark of life is to allow undead to take ability damage despite their lack of con score and type.

If dropping the creature's int to zero is not possible, drop it's dex to zero and leave it permanently paralyzed.

[Bohandas]

Liches are immune to polymorph, which I assume would also include petrification.
but those are not exactly liches, so it may be worth a try

According to The Complete Mage pg 9, only spells based on Polymorph or Alter Self or that are explicitly tagged as polymorph are considered polymorphing

[SirNibbles]

And further the general rule is overruled by the specific text of the spark of life spell. It doesn't matter why the undead is immune to ability damage. The spell spark of life explicitly undoes that immunity regardless of the immunity arising from the Undead type or a lack of a con score. It is clear that the intent of spark of life is to allow undead to take ability damage despite their lack of con score and type.

If dropping the creature's int to zero is not possible, drop it's dex to zero and leave it permanently paralyzed.

Yes, hence my recommendation of that spell.

[Bohandas]

Note, also, that certain spells apply penalties to ability scores rather than actually damaging them (although the two that spring most readily to mind, Bestow Curse and Ray of Enfeeblement can't reduce a score to zero. They can reduce it down to 1 though)

[Chainguy]

A creature/captor with the Touch of Golden Ice feat could keep them at 0 dex indefinitely - then have them wear metal armor to prevent Wild shape. Being immobilized and unable to use druid spells and supernatural abilities should keep them down for a while(?).

[Beldar]

The metal armor/shield suggestion is a good one. If the Dm gives you grief about that not violating their druid oath, then you could try Command Undead or Control Undead to have them pick it up or wear it deliberately and see if that satisfies him. That depends a lot on him.

So here is another idea.
Get an iron coffin.
Get one of the undead into it, by whatever means (Control Undead spell, grapple, whatever).
Have the wizard standing by with a readied action, and pre-positioned materials, and when the moment is right, cast Metal Melt (4th or 5th level), filling the coffin with liquid metal that isn't hot.
Not being hot, it doesn't kill the lich and let him go re-form at his phylactery.
Being liquid (temporarily) the metal *exactly* forms to the shape of the lich and encases him. Then it hardens back into normal iron.

Problem solved. They can't move at all, not even a millimeter, so no somatic or even verbal spells can be cast (the jaw can't move), nor any material components either.
He can wild shape into something that can fit within the lich-shaped cavity in the solid, seamless, block of iron, but that isn't going to help him because, done right, the metal is both much thicker than anything that could be bent or broken by mere muscles, and it is airtight and water tight.
His elemental forms of air and water can't flow out of it. His earth form can't move through it, since it isn't stoone or earth. And his fire form can't melt it, since it is both thick, and you will have sunk it to the bottom of a lake or ocean, so any heat applied to the metal gets conducted to the water, and carried away by convection. He'd have to be able to boil the whole lake to melt the metal.
No elemental I'm aware of can generate enough heat to boil a lake, much less an ocean.

In a few centuries or longer, the metal will eventually rust away. But by then you will be long gone (and might have found and destroyed the phylactery in the meantime).

[King of Nowhere]

The metal armor/shield suggestion is a good one. If the Dm gives you grief about that not violating their druid oath, then you could try Command Undead or Control Undead to have them pick it up or wear it deliberately and see if that satisfies him. That depends a lot on him.


if being forcibly strapped to a piece of equipment does not violate oaths, then i fail to see how doing it under compulsion would be any different.
It's quite moot anyway, as our wizard has banned school necromancy.

the rest of your post is quite nice on the other hand. encasing them in metal would be viable.

[bekeleven]

I have a guide on preventing resurrection (https://forums.giantitp.com/showthread.php?419197-And-Stay-Out-A-Guide-to-Staying-Dead-(for-the-other-guy)). Many of the tricks, though obviously not all, of the tricks can be used to prevent a Lich from Phylactarying.

For instance:

Sequester is Will Negates (Object) to put a creature into suspended animation for 1 day/level.
If you lead with Spark of Life, you can throw a Touch of Juiblex, Smoky Confinement, or other spell to permanently trap them.

[Fouredged Sword]

The metal armor/shield suggestion is a good one. If the Dm gives you grief about that not violating their druid oath, then you could try Command Undead or Control Undead to have them pick it up or wear it deliberately and see if that satisfies him. That depends a lot on him.

So here is another idea.
Get an iron coffin.
Get one of the undead into it, by whatever means (Control Undead spell, grapple, whatever).
Have the wizard standing by with a readied action, and pre-positioned materials, and when the moment is right, cast Metal Melt (4th or 5th level), filling the coffin with liquid metal that isn't hot.
Not being hot, it doesn't kill the lich and let him go re-form at his phylactery.
Being liquid (temporarily) the metal *exactly* forms to the shape of the lich and encases him. Then it hardens back into normal iron.

Problem solved. They can't move at all, not even a millimeter, so no somatic or even verbal spells can be cast (the jaw can't move), nor any material components either.
He can wild shape into something that can fit within the lich-shaped cavity in the solid, seamless, block of iron, but that isn't going to help him because, done right, the metal is both much thicker than anything that could be bent or broken by mere muscles, and it is airtight and water tight.
His elemental forms of air and water can't flow out of it. His earth form can't move through it, since it isn't stoone or earth. And his fire form can't melt it, since it is both thick, and you will have sunk it to the bottom of a lake or ocean, so any heat applied to the metal gets conducted to the water, and carried away by convection. He'd have to be able to boil the whole lake to melt the metal.
No elemental I'm aware of can generate enough heat to boil a lake, much less an ocean.

In a few centuries or longer, the metal will eventually rust away. But by then you will be long gone (and might have found and destroyed the phylactery in the meantime).
Use lead. Now they are also protected from most forms of scrying because lead blocks that.

[Quertus]

Use lead. Now they are also protected from most forms of scrying because lead blocks that.

Lead is soft, and melts easily.

[Goaty14]

Lead is soft, and melts easily.

Still a good shoutout to prevent divinations from reaching it. Very easily you could get lead plating to cover the exterior/interior of the coffin.

[InvisibleBison]

The metal armor/shield suggestion is a good one. If the Dm gives you grief about that not violating their druid oath, then you could try Command Undead or Control Undead to have them pick it up or wear it deliberately and see if that satisfies him. That depends a lot on him.

So here is another idea.
Get an iron coffin.
Get one of the undead into it, by whatever means (Control Undead spell, grapple, whatever).
Have the wizard standing by with a readied action, and pre-positioned materials, and when the moment is right, cast Metal Melt (4th or 5th level), filling the coffin with liquid metal that isn't hot.
Not being hot, it doesn't kill the lich and let him go re-form at his phylactery.
Being liquid (temporarily) the metal *exactly* forms to the shape of the lich and encases him. Then it hardens back into normal iron.

Problem solved. They can't move at all, not even a millimeter, so no somatic or even verbal spells can be cast (the jaw can't move), nor any material components either.
He can wild shape into something that can fit within the lich-shaped cavity in the solid, seamless, block of iron, but that isn't going to help him because, done right, the metal is both much thicker than anything that could be bent or broken by mere muscles, and it is airtight and water tight.
His elemental forms of air and water can't flow out of it. His earth form can't move through it, since it isn't stoone or earth. And his fire form can't melt it, since it is both thick, and you will have sunk it to the bottom of a lake or ocean, so any heat applied to the metal gets conducted to the water, and carried away by convection. He'd have to be able to boil the whole lake to melt the metal.
No elemental I'm aware of can generate enough heat to boil a lake, much less an ocean.

In a few centuries or longer, the metal will eventually rust away. But by then you will be long gone (and might have found and destroyed the phylactery in the meantime).
This will keep the lich imprisoned for at most a day - they'll shapeshift into a form small enough that they can cast a spell in the cavity, wait until the next time that they prepare spells, and then prepare rusting grasp (http://www.d20srd.org/srd/spells/rustingGrasp.htm) in all their spell slots.

Edit: And making the prison out of a non-ferrous metal won't help - transmute metal to wood (http://www.d20srd.org/srd/spells/transmuteMetalToWood.htm) followed by wood shape (http://www.d20srd.org/srd/spells/woodShape.htm) also allows for escape.

[Beldar]

True, if they can cast spells, they can get out - there are so many spells, which can be used in imaginitive ways, I doubt we could ever keep them contained - IF they can cast.

And true, if they can find a shape to wild-shape into, which is both small enough to move freely within the cavity, AND can cast without the Natural Spell feat (or if they happen to have that feat), then they could arguably cast spells within the cavity that would get them out.
Some of that is up to DM interpretation. Some of it is up to chance (ie, do they happen to have Natural Spell, or some other feats that'd make the difference, like a combination of Still Spell and Silent Spell.)

So we can't know for sure in advance that it'd work.
But the nice thing is that we don't have to.

This is not a rocket launch for a moon landing - we don't have to get it perfect on the first try. We can try it, see if it works, then modify and try again if needed.
"Peel the onion" - defeat x, y, z, discover that he can still do a, then in the next try, include a way to defeat a as well.

Remember that the problem we're trying to solve here is that the druid/liches keep pestering the party. We win (achieve our goal on stopping the pestering) whether A) we seal them away permanently, OR B) they get concerned that we may achieve that result and so stay away from us.

And as long as they keep coming and pestering the party, the party can keep trying new things.

So, if they rust away their containment block and come back (or if you think of it in advance - whatever), then your next try includes an attempt to keep them from spellcasting.
There are options.
Maybe you do your research, find that Druids can only go as small as Tiny (cat sized basically), and you shrink him that small *before* you seal him in his containment block (such as by Baleful Polymorph).
Now his containment block has only a Tiny-sized cavity it in, and he can't go smaller than that, so he should be unable to do somatic and verbal components.

For good measure, we throw in another good idea mentioned here and soveriegn glue him into a Tiny-sized suit of full plate fitted to whatever shape we polymorphed him into.
Why?
Well, oath-questions aside, if he manages to wild shape in some way that gives him some wiggle-room, he now has a suit of full plate (or chainmail or whatever works out best) drooping within the cavity and further constraining his space.
So much is up to the dm here that we can never be sure, but it is still worth a try.
For instance, the DM may decide that, though the druid didn't choose voluntarily to wear metal armor, he still can't cast while he has it on. And, in the constrained space within his confinement block, he can't really take it off (how can it really be "off" if there is no place to put it other than around yourself and still in contact with yourself).

As for the druid oath about metal armor - since you fail to see the difference, I will point it out.
The difference between having it glued to you, and putting it on yourself while under compulsion, is the difference between "I didn't do it, it was done to me" and "I did it but was under duress". Admittedly, that isn't much difference, and I tried to acknowledge that, but in some contexts, even that small difference matters - they have to draw the line somewhere, and, depending on who draws it, it may be there. ie, it depends on the DM, so may be worth a try.
A number of cases in court have been decided on smaller differences
For instance, at the Nuremburg trials, those who could prove they didn't do it got a LOT different result than those whose best excuse was that they were under duress. It isn't a perfect example by any means, since D&D method's of duress are more effective, but it can illustrate the point.

Anyway, back to more practical matters, since "hair splitting for fun and profit" isn't actually fun.

What if the druid/liches defeat the new and improved containment block (for example, maybe he has a prestige class dip that gives him another way out, such as Diminutive forms)?
We try other things, and keep trying until we find one that works, or he stays away.

For example, we can add, to his metal armor, a resetting magical trap of Silence or whatever other spell we think will prevent him from casting (ideally a low-level spell to keep the cost down, but whatever).

We should also probably put in there a small amount of Thinaum with him, in case he manages to suicide - if he does, the Thinaum will absorb his soul and hopefully keep it from reforming via phylactery.
Is that perfect? No, the way the Thinaum description is written, it assumes you will make a weapon out of it, and we're not doing that here.
So again, something is up to the DM, to decide whether our usage fits within the spirit of the rule of that or not.

Other thoughts -

Adding lead is a good idea, but I'd put a layer on the outside of the containment block, since lead is soft enough it can be defeated by fingernails - he can claw his way out eventually.

Another interesting trick, costly and imperfect, but it could really put some fear into them:
Since you have 9th level spells available. From the Book of Exalted Deeds, cast Sanctify the Wicked on them.
On a failed Will save, it would get the druid/liches (one per spell) locked away for a year (and not pestering you during that time, though you should guard the gem they are in), and at the end of the year they change alignment to match yours.
Even if they succeed at the will save, they will probably recognize what you attempted, and may be so horrified by the idea of changing alignments that they stay away just to prevent you from trying again.

[Quertus]

@Beldar, you forgot, "we lose if they learn that we are exploiting their vulnerability to X, and resolve that issue".

So, if we try X, Y, Z, and it fails because Z, and they say, "wow, good thing they didn't try XKDC", and invest in becoming immune to X, then you've lost the element of surprise, lost the initiative.

So, this might be "the moon landing", might be something where you have to land it in one. Or it might not. We don't know.

But, if you try it on the Quertus of liches, you'd best get it right the first time. Or the third+ time, because Quertus will likely have forgotten about it by then…

[Beldar]

Quertus: Yes, against hypothetical, arbitrary foes, it's a different question, as you say.

But the original poster made it clear that these druid/liches are not a serious threat to the party.
"they are no match for us in direct combat"
"capturing some alive is trivial enough"
and similar statements.

So that is the context in which I was attempting to answer. Not the context of "what if the liches are optimized", which seems to be what you're suggesting.

Also, we do not lose if they come up with an answer and escape - that just sends us back to square 1 - where they are "pestering us", no worse than if we hadn't tried at all.

[Quertus]

Quertus: Yes, against hypothetical, arbitrary foes, it's a different question, as you say.

But the original poster made it clear that these druid/liches are not a serious threat to the party.
"they are no match for us in direct combat"
"capturing some alive is trivial enough"
and similar statements.

So that is the context in which I was attempting to answer. Not the context of "what if the liches are optimized", which seems to be what you're suggesting.

Also, we do not lose if they come up with an answer and escape - that just sends us back to square 1 - where they are "pestering us", no worse than if we hadn't tried at all.

You misunderstand. Let me try again, this time by way of parable.

So, say that there's someone I can always beat at… a race, why not. I decide that I want to… super beat them by… taking a shortcut that they don't know. After I take this shortcut, they *might* learn of the existence of this shortcut, and add it to their skill set. So, if there's something I'd really like to pair with beating them extra (like, wearing a certain shirt, or having certain music playing), I *may* need to set that up the first time. Or I might not - I *might* get other chances before they learn this trick.

Point is, the fact that you always beat them does not mean that they are incapable of learning. In fact, IME, beating someone a new way is often one of the better incentives for them to improve.

It's not that they suddenly become a threat, simply that they may develop a counter to some component(s) of the plan.

It has nothing to do with optimization, and everything to do with… sentience? Outlook? Adaptability? Their ability to learn from experience.