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View Full Version : Purified Explosive Runes... cheating or awesome?



DaFlipp
2007-05-08, 04:04 PM
So, my gnome spellcaster enjoys the spell Explosive Runes. As a rule, he has it memorized every day, and casts it at the end of the day on a piece of paper before he goes to sleep - tacking on Sudden Empower and Sudden Maximize if he hasn't used those yet throughout the day. Thanks to downtime, he has a pretty hefty supply of notes featuring Maximized, Empowered Explosive Runes. While these would wreak havoc in the hands of, say, a Chaotic Neutral rogue, my neutral good mage is judicious enough not to use these willy nilly.

But with 4th level spell slots comes great responsibility... er, I mean, an interesting opportunity. See, I gave him the Purify Spell feat from Exalted Deeds - which lets you bump the spell slot up one, in exchange for the spell dealing no damage to Good creatures (and half-damage to neutral, and extra damage to Outsiders).

So. Purified Explosive Runes. Purified Maximized Empowered Explosive Runes.

Is there a rule I'm breaking that I'm not aware of, or do I now have a (thanks to downtime) near-endless supply of 36+3d6 damage notes that anyone in my all-Good party can read at any time to damage everything evil surrounding them?

(My DM doesn't seem to mind it, if only because it tends to encourage more strategic battles than hack-and-slash fests. My party used a combination of Purified Explosive Runes and Hide From Undead recently to topple a group of undead that would've taken hours to fight conventionally.)

Darth Mario
2007-05-08, 04:14 PM
Not cheating, brilliant, and if your DM doesn't mind it, keep going! Just be warned, when he gets sick of it, you will no doubt be faced with a group of Good/Neutral characters that have some qualm with you. Make sure to keep some non-purified explosive scrolls on hand. (Your hands only, your party members will undoubtedly confuse them with the pure ones.)

Wonderful trick!

brian c
2007-05-08, 04:15 PM
So, my gnome spellcaster enjoys the spell Explosive Runes. As a rule, he has it memorized every day, and casts it at the end of the day on a piece of paper before he goes to sleep - tacking on Sudden Empower and Sudden Maximize if he hasn't used those yet throughout the day. Thanks to downtime, he has a pretty hefty supply of notes featuring Maximized, Empowered Explosive Runes. While these would wreak havoc in the hands of, say, a Chaotic Neutral rogue, my neutral good mage is judicious enough not to use these willy nilly.

But with 4th level spell slots comes great responsibility... er, I mean, an interesting opportunity. See, I gave him the Purify Spell feat from Exalted Deeds - which lets you bump the spell slot up one, in exchange for the spell dealing no damage to Good creatures (and half-damage to neutral, and extra damage to Outsiders).

So. Purified Explosive Runes. Purified Maximized Empowered Explosive Runes.

Is there a rule I'm breaking that I'm not aware of, or do I now have a (thanks to downtime) near-endless supply of 36+3d6 damage notes that anyone in my all-Good party can read at any time to damage everything evil surrounding them?

(My DM doesn't seem to mind it, if only because it tends to encourage more strategic battles than hack-and-slash fests. My party used a combination of Purified Explosive Runes and Hide From Undead recently to topple a group of undead that would've taken hours to fight conventionally.)

Well, Purified, Maximized, Empowered Explosive Runes would require a... what, 8th level slot?

That said, I think the tactic itself is valid and very clever. Good job.

DaFlipp
2007-05-08, 04:17 PM
Well, Purified, Maximized, Empowered Explosive Runes would require a... what, 8th level slot?

That said, I think the tactic itself is valid and very clever. Good job.

He has the Sudden Maximize and Sudden Empower feats - those allow you to modify a spell WITHOUT changing its slot level, but you can only do so once per day. They're in Complete Arcane.

I looked at them very carefully, and saw no note about them not being allowed to stack with other metamagic, Sudden or otherwise...

Penguinsushi
2007-05-08, 04:33 PM
Of course, the 'Purify Spell' wouldn't work on the ones you've already written, but yes - that's brilliant!

Explosive runes is one of those (many) spells I've not taken time to learn the finer points on - so I'd never have thought of that.

Of course, it's much more difficult in a mixed party, but yeah, and all-good party could wreak some havoc on the evil. :smallsmile:

~PS

Ulzgoroth
2007-05-08, 05:06 PM
The only thing wrong with it is the existence of Purify Spell metamagic...one more for the 'bad ideas in BoED' list... But very clever use of it, especially if your enemies are consistently evil.

Beware area dispels. Doubly so for the old, non-purified runes!

ChomZ
2007-05-08, 05:45 PM
That a pretty awsome 'get out of my face' evil repelant.. explosive ruins just keep getting more fun

Magi_Ring_O
2007-05-08, 05:58 PM
That is a great idea. I just hope my DM doesn't see this - we are playing an evil campaign right now, and he is just the sort to do this sort of thing.

Emperor Tippy
2007-05-08, 06:15 PM
Excellent use of the spell and feats. I may have to steal that for the church in one of my games.

martyboy74
2007-05-08, 06:18 PM
You do realize that this will still hurt everything within a 20' radius of you? The church wouldn't do it because it would still hurt innocent bystanders.

Emperor Tippy
2007-05-08, 06:26 PM
The church in this game is very much of the belief that anything that isn't actually good should be killed.

So they would have no problem using this in the middle of the market. If any bystanders die well they weren't good so its all good.

SpiderBrigade
2007-05-08, 06:29 PM
Only bystanders who aren't Good! Depending on the church they might not care too much about Neutral guys.

Yah, the other thing to do with the old, non-Purified ones is to stack them all together in a box and have the rogue or somebody UMD a wand of low-level Dispel, thereby failing to dispel them...and setting off all thousand pages of runes. Small blast radius, but all SORTS of damage.

Emperor Tippy
2007-05-08, 06:42 PM
A great use for them (if you can make custom magic items) is to make a 1 time use box of dispel that hits everything inside of it with a targeted dispel at minimum CL. Its activation condition is a pin being pulled from the side of it and then throwing it (at a specific square).

You now have force grenades powerful enough to decimate everything within a 10 foot radius. But something 11 feet away doesn't even notice the explosion.

ocato
2007-05-08, 08:02 PM
Our party wizard used to love this trick. Put it on the Paladin's shield and watch him fly through the front lines.

tsuyoshikentsu
2007-05-08, 08:03 PM
If any bystanders die well they weren't good so its all good.

I should hope it were, after 3,287,241 purified explosive runes. *Ba-dum-pish*

Jasdoif
2007-05-08, 08:11 PM
Beware area dispels. Doubly so for the old, non-purified runes!Yes, area dispels are the big drawback to the tactic. Explosive Runes are a spell effect, not a magic item, so a dispel check is made against each and every rune-bearing note you have. And Explosive Runes has a particular behavior to it: on failing a dispel check, the runes detonate.

So a single area dispel magic of any caster level and all the explosive runes are gone.

Emperor Tippy
2007-05-08, 08:18 PM
Thats why you store them in a Bag of Holding.

Or only keep Purified Explosive Runes around. If they go boom then your fine.

Indon
2007-05-08, 08:32 PM
Thats why you store them in a Bag of Holding.

Or only keep Purified Explosive Runes around. If they go boom then your fine.

I'd go with storing all but one or two, even the purified one. Never know when you might end up escorting an NPC who just happens to be LN.

Darth Mario
2007-05-08, 08:41 PM
Eventually your DM will get sick of this trick (I know I would after about a week) and will come up with some way to get around it (if he really hates it, he'll use dispel as previously mentioned). I agree with those advocating storing them in a bag of holding, I'd say keep all but 2 purified ones in there.

Fishy
2007-05-08, 09:04 PM
I don't know if I'd give it to players, but Purify Spell combined with a massive, massive explody effect seems like a wonderful plot hook for a BBEG. Or, um, GG, as the case may be.

We'd start small, then metamagic our area of effect up to the size of a city; then unpleasent heretical nations, then the ENTIRE PLANE!!

knightsaline
2007-05-08, 09:12 PM
We'd start small, then metamagic our area of effect up to the size of a city; then unpleasent heretical nations, then the ENTIRE PLANE!!

Reset...THE WORLD!!!!!

Yvian
2007-05-08, 09:16 PM
The only thing wrong with it is the existence of Purify Spell metamagic...one more for the 'bad ideas in BoED' list... But very clever use of it, especially if your enemies are consistently evil.

Beware area dispels. Doubly so for the old, non-purified runes!

First, yes brillent.

Second - Ulzgoroth - why is purify wrong? I would like to hear your arguments.

DaFlipp
2007-05-08, 10:08 PM
Hmmm, thanks for the heads-up on the possibility of self-explodey during an area dispel check. I should ditch those soon... maybe I'll toss the entire bag at the next highly-intimidating creature we meet, cast Dispel Magic, and watch my DM's eyes cross.

Purify Spell is useful, indeed. Even if you aren't a fan of Exploding Runes, who wouldn't love a Fireball or similar area-of-effect spell that won't turn your allies crispy? (At least the Good ones...)

Ulzgoroth
2007-05-08, 10:21 PM
Second - Ulzgoroth - why is purify wrong? I would like to hear your arguments.
Well, first of all I don't buy the idea of a fireball that peers into your soul and decides how much to crisp you. Regardless of anything else. Spells that do that type of thing are strongly based on forces that have reason to be selective. Secondly, it's available to all casters. How many core sor/wiz spells have anything to do with alignment? I can think of the 'protection from x' cycle and the related 'magic circle against x' cycle. That's all I can come up with. Arcane magic has a very weak grip on alignments, and now we've got wizards (who can't cast detect evil!) throwing splash damage that reads them on the fly? Um, no.

That's the theory, but the practice is bad too. First there's this little gem, that disposes of the one good reason it's unhealthy and challenging to use Explosive Runes as all-purpose antipersonnel bombs. That alone would convince me to put it out (not that using it this way isn't awesome, I just don't think it should be supported by the rules). But even in general, what's this metamagic for? Dropping explosive spells wherever you feel like without having to worry about what's in the area, right? That doesn't seem like a good thing, nor does it seem affiliated with Good.

Jasdoif
2007-05-08, 10:24 PM
Purify Spell gives you and any other good characters blanket immunity to anything you cast that does damage. In general it makes area effect spells much more versatile because you don't need to care if you or your allies are in the area. Offensive spells becomes defensive.

Flame Strike yourself. Cloudkill yourself, and walk in the middle of it. Horrid Wilting yourself. Wall of Fire yourself. Cast Disintegrate into a grapple. Cast Lightning Bolt through your party. Drop a Cone of Cold into melee. And of course, freedom from the balancing factor of mass Explosive Runes, as this thread is all about. All this and more, for the cost of one feat and bumping up spells by one level?

Oh, and take BoED's version of Spell Focus (good) and all the save DCs of these spells have a +2 bonus, too.

Latronis
2007-05-08, 11:10 PM
Purify Spell gives you and any other good characters blanket immunity to anything you cast that does damage. In general it makes area effect spells much more versatile because you don't need to care if you or your allies are in the area. Offensive spells becomes defensive.

Flame Strike yourself. Cloudkill yourself, and walk in the middle of it. Horrid Wilting yourself. Wall of Fire yourself. Cast Disintegrate into a grapple. Cast Lightning Bolt through your party. Drop a Cone of Cold into melee. And of course, freedom from the balancing factor of mass Explosive Runes, as this thread is all about. All this and more, for the cost of one feat and bumping up spells by one level?

Oh, and take BoED's version of Spell Focus (good) and all the save DCs of these spells have a +2 bonus, too.

That use of purify is cool, but i don't like the good vs evil part of it either. All it does is allow people to remove that aspect of roleplaying. And according to BoED almost everything is evil anyway.

Does it include ability damage?

Jasdoif
2007-05-08, 11:19 PM
Does it include ability damage?Well, it doesn't specify hit point or any other kind of damage, so I would assume it applies to all kinds of damage. The feat could really use some restrictions, or better wording, in order to make it less game-breaking or style-breaking.

Or even better (for balance), just use another feat altogether: Consecrate Spell's in BoED, and was reprinted in Complete Divine (although BoED's suggests that good creatures don't take the divine portion of the damage, while Comple Divine's only says that energy immunities/resistances don't apply to the divine portion).

Latronis
2007-05-08, 11:31 PM
Or ignore the book of exalted crap on principle

SpiderBrigade
2007-05-09, 06:32 AM
Eventually your DM will get sick of this trick (I know I would after about a week) and will come up with some way to get around it (if he really hates it, he'll use dispel as previously mentioned). I agree with those advocating storing them in a bag of holding, I'd say keep all but 2 purified ones in there.It really depends on how you use it, I think. How fast the DM gets so tired of the trick that he comes up with ways to stop it, I mean. If the entire party starts to ride the Explosive Rune Bomb as a sort of win-button gimmick, then yes. If the wizard just uses it as part of his repertoire of spell tactics, I don't think it'd be that big of a problem. Certainly not when compared to any of the other things a high-level caster can be doing.

NullAshton
2007-05-09, 07:10 AM
A high DC glitterdust into the party will stop it. How can they read the purified explosive runes if they're blind? The ones who will actually succeed on the will save will probably be squishy targets anyway.

Emperor Tippy
2007-05-09, 07:24 AM
A high DC glitterdust into the party will stop it. How can they read the purified explosive runes if they're blind? The ones who will actually succeed on the will save will probably be squishy targets anyway.
To set off an explosive rune you can just rip the paper holding it in half. It counts as an erase attempt, thus setting off the rune.

ZeroNumerous
2007-05-09, 08:05 AM
Make all your Explosive Runes in braile. Then you and all your friends put two skill points in Speak Language: Braile. You can't speak in dots, true, but you automatically read/write any language you can speak. Blinding the PCs no longer matters.

DaFlipp
2007-05-10, 01:43 AM
...Hmm.

Would setting an Explosive Rune paper on fire count as an "erase" attempt?

*throws bag at enemy, casts Scorching Ray on the -bag-*

kpenguin
2007-05-10, 01:51 AM
Of course, explosive runes deals fire damage. If any of your allies are immune to fire, you don't even to purify the spell. If you have the energy substitution and your allies are immune to something else, you can change it to that energy type with no spell slot added.

Jasdoif
2007-05-10, 01:55 AM
Of course, explosive runes deals fire damage.No, explosive runes (http://www.d20srd.org/srd/spells/explosiveRunes.htm) deal force damage. Slightly odd, isn't it?

kpenguin
2007-05-10, 01:58 AM
Huh... explosive runes deal force damage? If not fire, I would expect it to just to be bludgeoning damage from the concussive force.

ZeroNumerous
2007-05-10, 07:44 AM
Nope. Pure force. Besides that, you don't apply DR to magical attacks anyway.

Rad
2007-05-10, 09:12 AM
I feel like this is exactly the same thing as Xykon's rubberball... the use of a trap spell as an offensive one.
According to RAW, the trick works great and I am sincerely admiring the guys who came up with it. On the gaming enjoyment side I have a couple of concerns. If I were the DM I would houserule all the trap-like spells to deal damage only to the allies of the creature triggering the spell. This is an house rule though.

Illiterate Scribe
2007-05-10, 12:24 PM
Just more indication the Explosive Runes = teh awesome.

Ooh, I just noticed it's level 3. Within eternal wand reach. Within reach of Artificer. Within reach of Metamagic Trigger. Oh dear.

YOUR HEAD A'SPLODE.

DaFlipp
2007-05-10, 01:53 PM
Just more indication the Explosive Runes = teh awesome.

Ooh, I just noticed it's level 3. Within eternal wand reach. Within reach of Artificer. Within reach of Metamagic Trigger. Oh dear.

YOUR HEAD A'SPLODE.

Er, I'm still kind of a new hand at optimization, and don't have access to all the books. What are all those things you're mentioning?

Tokiko Mima
2007-05-10, 02:04 PM
This goes right along with the idea of using the poisons (Unicorn Blood, Golden Ice, etc) from BoED in the village well. Suddenly all those greedy evil merchants and vampires/demons in disguise fall sick, and you know who to burn at the stake. Good aligned folks suffer no ill effects at all. It's the ultimate witch tester! Pretty cheap way to root out the evil elements in your society. :smalltongue:

Telonius
2007-05-10, 02:14 PM
It's been awhile since I've read Exalted Deeds. IIRC, you have to maintain not just a good alignment, but be exalted to use most of the stuff in the book. Creating something with the possibility of high collateral damage doesn't sound all that exalted to me.

Otherwise, brilliant.

Fizban
2007-05-10, 02:21 PM
You do for Exalted feats, but not for metamagic feats.

Kiroho
2007-05-10, 02:42 PM
And using the results of any Purified offensive spell is an interesting way for the DM to "reward" the all-good party members that are straying from their alignment.

Joe: I cast a Purified Fireball into melee.

DM: OK, the BBEG bursts into flames and runs screaming about the room in pain. Dan, roll a save.

Dan: WHAT!?!? Why? I'm good, I'm immune to the fireball.

DM: Remember that barmaid you stiffed for the bill last night at the tavern . . . . .


OK, maybe a bit extreme, but you get the point.

Corolinth
2007-05-10, 03:06 PM
I think it's a delightfully gnomish idea. Kudos.

Now, this is one of the reasons why I only allow feats from non-core books on a case by case basis. That being said, there's plenty of ways for your DM to deal with this. If you're carrying too many notes, expect to be hit by a fireball and have to start over from scratch.

ZeroNumerous
2007-05-10, 03:32 PM
Er, I'm still kind of a new hand at optimization, and don't have access to all the books. What are all those things you're mentioning?

You don't wanna know. It's from Eberron, but please. You don't wanna know.



Joe: I cast a Purified Fireball into melee.

DM: OK, the BBEG bursts into flames and runs screaming about the room in pain. Dan, roll a save.

Dan: WHAT!?!? Why? I'm good, I'm immune to the fireball.

DM: Remember that barmaid you stiffed for the bill last night at the tavern . . . . .


That just makes him non-Lawful.

MeanJoeSmith75
2007-05-10, 03:48 PM
I'm not clear on Purify Spell Feat as I don't our Exhalted, but as a GM I would love to have my PCs try this. Explosives effect more than just living things. Use it in a mine and the walls come tumbling down. That nice chest of gold behind the enimies...oops there goes the treasure. Maybe you can get a shopvac to suck the gold dust out of the air. Oh yeah. The centered on you thing....ever here of oxygen depression. That much explosive force could easily push all the air way(suffocation) then the rush back(thunderclap)

Jasdoif
2007-05-10, 03:53 PM
Explosives effect more than just living things.The spell description states it damages creatures within 10 feet, and the object the runes are inscribed on. I have a hard time believing it affects other objects, though I can see some potential ambiguity in the wording.

MeanJoeSmith75
2007-05-10, 04:00 PM
I would consider this a bit of rules jockeying. The effect is clearly a force effect ands there is no logical reason why it would not effect unattended items similar to fireball. Just be cause the description explicitly states creatures, doesn't mean it explicitly excludes objects.

Tokiko Mima
2007-05-10, 04:05 PM
I would treat it like they were using the Mastery of Shaping feature from Archmage for the purposes of the spell in question, and that it automatically excluded the square of any good aligned person in the spells radius.

ZeroNumerous
2007-05-10, 04:06 PM
Actually it sort of does. Many other spells specify only functioning versus objects, or not functioning versus objects. As such, I'd expect that a spell that specifies function versus creatures to exclude function versus objects.

MeanJoeSmith75
2007-05-10, 04:10 PM
Magic missle explicitly says it doen't damage objects. however, the fact the explosive runes is an area effect vs taget would point to a more kinetic force. Again, its a bit of rules interpretation and as GM of my game I would rule it that way. Results may vary.

Jasdoif
2007-05-10, 04:10 PM
The effect is clearly a force effect ands there is no logical reason why it would not effect unattended items similar to fireball. Just be cause the description explicitly states creatures, doesn't mean it explicitly excludes objects.Just because it doesn't explicitly exclude objects doesn't mean it implicitly includes objects, either. That fireball you mentioned explicitly states it damages unattended objects.

If explosive runes were meant to damage unattended objects, it should have said it damaged creatures and unattended objects within 10 feet, instead of just creatures with 10 feet.

MeanJoeSmith75
2007-05-10, 04:29 PM
The Magic Missle description explicitly excludes object. These forces are ment to be interpreted. I would interpit a normal Explosive Runes to be in effective against sturdy, solid objects. The more extreme examples I noted above are for when some one tries the old 100 pieces of parchment each with Explosive Runes cast on them. Scale of explosion changes so does the effects. Now on to the force effects. If it were psy runes or even negative energy runes I would leave non-living things out of it, but my rule of thumb is, if it can effect a golem, it can effect an object.

Fax Celestis
2007-05-10, 05:00 PM
Huh... explosive runes deal force damage? If not fire, I would expect it to just to be bludgeoning damage from the concussive force.

Explosions aren't firey; they're an outward-bound concussive force. Therefore, force damage.


ex·plo·sion /ɪkˈsploʊʒən/ Pronunciation Key - Show Spelled Pronunciation Pronunciation Key - Show IPA Pronunciation
–noun 1. an act or instance of exploding; a violent expansion or bursting with noise, as of gunpowder or a boiler (opposed to implosion).
2. the noise itself: The loud explosion woke them.
3. a violent outburst, as of laughter or anger.
4. [I]a sudden, rapid, or great increase: a population explosion.
5. the burning of the mixture of fuel and air in an internal-combustion engine.

Explosions usually involve fire, but not always.

Collin152
2007-05-10, 10:02 PM
This thread is made of win. Just send me a couple bags of those Purified Runes, and put a Sepia Snake Sigil on the stamp.

Citizen Joe
2007-05-10, 11:57 PM
from SRD
"The object on which the runes were written also takes full damage (no saving throw)."

Thus at least the object is likely to be damaged if not destroyed (6d6 damage).

kellandros
2007-05-11, 01:18 AM
"The object on which the runes were written also takes full damage (no saving throw)."

Since this spell just takes a standard action, does this mean you could inscribe this onto people's armor/golems, assuming spell resistance doesn't kick in?

The combat calligrapher is born!!

Citizen Joe
2007-05-11, 01:24 AM
Since this spell just takes a standard action, does this mean you could inscribe this onto people's armor/golems, assuming spell resistance doesn't kick in?

The combat calligrapher is born!!

From SRD "You trace these mystic runes upon a book, map, scroll, or similar object bearing written information." So, no...

Aethir
2007-05-11, 01:45 AM
This makes me glad I can use the Runesmith class from Races of Stone :smallamused:

Turcano
2007-05-11, 02:50 AM
This goes right along with the idea of using the poisons (Unicorn Blood, Golden Ice, etc) from BoED in the village well. Suddenly all those greedy evil merchants and vampires/demons in disguise fall sick, and you know who to burn at the stake. Good aligned folks suffer no ill effects at all. It's the ultimate witch tester! Pretty cheap way to root out the evil elements in your society. :smalltongue:

Well, it's a damn sight better than judging guilt by one's failure to drown...

ZeroNumerous
2007-05-11, 09:42 AM
From SRD "You trace these mystic runes upon a book, map, scroll, or similar object bearing written information." So, no...

So you stick a piece of paper to the golem/armor first. Done.

Indon
2007-05-11, 09:48 AM
So you stick a piece of paper to the golem/armor first. Done.

Easier just to attach a prewritten explosive rune, methinks.

Jayabalard
2007-05-11, 11:28 AM
If all you want is for your allies to be able to read them, why do you need the purification? Just instruct them as to how to read the writing without triggering the runes...
You and any characters you specifically instruct can read the protected writing without triggering the runes. Likewise


To set off an explosive rune you can just rip the paper holding it in half. It counts as an erase attempt, thus setting off the rune.Is there some text on this other than what's in the SRD?

Another creature can remove them with a successful dispel magic or erase spell, but attempting to dispel or erase the runes and failing to do so triggers the explosion.
it seems pretty clear to me that the attempting to erase the runes is talking about the erase spell effect, just like dispel would be talking about the dispel magic spell effect. So I don't see how tearing it would trigger it.

The spirit of the spell seem to be (at least to me) to protect information or writing by preventing people from being able to read it. Tearing up or burning that information wouldn't set it off, and ruling it that way seems to be violating both the letter and spirit of the spell

Emperor Tippy
2007-05-11, 11:37 AM
When the SRD or item descriptions are referrign to a specific spell instead of an action that may have the same name they use italics.

If the description said erase then yo uwoudl be correct. But as written any attempt to erase the runes triggers them.

Erase is defined as


to rub or scrape out, as letters or characters written, engraved, etc.; efface.
2. to eliminate completely: She couldn't erase the tragic scene from her memory.
3. to obliterate (material recorded on magnetic tape or a magnetic disk): She erased the message.
4. to obliterate recorded material from (a magnetic tape or disk): He accidentally erased the tape.

I think that destruction counts as erasure.

But I coudl see ruling differently.

Fizban
2007-05-11, 02:50 PM
If all you want is for your allies to be able to read them, why do you need the purification? Just instruct them as to how to read the writing without triggering the runes...

No, he wants his allies to set off the runes. What he doesn't want is for them to take damage when they do so. So, ally takes out rune, reads, explosion deals damage to everyone within 10'. With Purify Spell, his good people are immune, and so can set off the runes without hurting themselves.