Low Level casters - High Level encounters

[MrStabby]

A couple of sessions ago I had my players face off against a campaign lieutenant. This guy was a pretty high level caster with a good number of meat minions in the encounter. I have seen this kind of thing a lot - the smart wizard with the dumb bodyguards.

I was thinking it could be fun (if more intensive bookkeeping) to antagonist who is a big tough melee boss, but is supported by low level caster minions.

The problem with this is that often the caster classes are too low on HP to not go down too quickly. One fireball can take out four or five level 3 wizards.

To work around this, whilst still keeping with the theme I wanted to work out what might be the best level 1 and level 2 spells vs a mixed party of adventurers.

My shortlist:
Bane (OK as it is likely to stick and can hit a lot of players. Downside is concentration so it can be stopped and the PCs will go from almost certainly hitting to almost almost certainly hitting)

Burning Hands (damage doesn't usually scale well but doing 3d6 damage to potentially a bunch of PCs isn't bad for a mook and an action. Its a save and not all players dex saves will be any higher than their lvl1 character)

Compelled Duel (not often a great spell but for a low level NPC to potentially take a much higher level PC out of the fight for a turn or so whilst they run away from the rest of the pack to deal with this guy. Bonus points for getting PCs to run over traps or to waste more time due to difficult terrain. Also see spike growth)

Dissonant whispers (Similar function to the duel, but a shorter more intense spell. It allows opportunity attacks and by front loading its effects you don't worry when the caster dies)

Blindness/Deafness (no concentration is good, but given the low level and limited number of spell slots this is less of a benefit)

Arcane lock (maybe can't be used that often, but cutting off one half of the party behind a tough door might be very effective)

Darkness (I include this simply for the surprise element, whilst the antagonist side may be equally hindered the PCs not knowing a) the numbers of bad guys or b) that the main bad guy is in the room may lead to some sub optimal decisions. 15ft radius isnt much though)

Enlarge/Reduce (This can be a pretty hefty buff or nerf for a level 2 spell; having 4 or 5 casters that know it can enable the big melee guy to be a titan of destruction till the support casters get dispatched. Less good as an offensive spell simply because most classes that rely on strength also have good con saves.)

Heat Metal (nothing particularly special about the circumstances just such a good spell that it keeps its value. The (kind of) lack of any saves is a big plus when going up against higher level enemies so I suppose this counts))

Hold Person (Again, just another mean spell. Wisdom save or be out of the fight and be very vulnerable to damage)

Levitate (what would be a great control spell on just about any save but Con; typically poor con saves correlate pretty well with having worthwhile ranged attack options)

Mirror image (worth it if the NPC can pre-buff before the fight, else they are probably better off directly casting a spell. These NPCs are not strategically worth keeping alive)

Spike Growth (the more time the PCs spend trying to dash about and mop up low level mobs hindered by their slow speed the longer bad guys can be pounding on them. A large area of difficult terrain that may not be expected can have a pretty big impact)

Suggestion (OK, this is a bit different but laying out a few on these on the party before the final encounter could produce some chaos. (I was wondering if the recipient would know that the spell was a suggestion spell? Maybe an arcana check at the time the suggestion is made. I was thinking that if the spell was "When you meet evil Baron Von Fellevilo you should begin the encounter by throwing a fireball at him", said several hours in advance then the save would be made when the condition was met, not when the spell was cast)

Water Walk (having a bunch of ranged hostile NPCs on the surface of deep water supporting an encounter may make them a tougher threat to eliminate.)

Web (usual ways the spell is good, plus even a level 12 party can fail a Dex save on their melee guys)


So these are my shortlist spells for my level 3 casters. It looks like the class with the most options is probably Wizard. Even I am not prepared to go with a chorus of divination wizards all spamming portent dice to force failed saves (plus the bookkeeping would be horrific). Possibly Enchantment, with Hypnotic gaze could give some useful stalling abilities to the casters for after they run out of spellslots.


Would people think this could be a fun basis for an encounter - a powerful melee character (roughly equivalent to a level 16 PC), 4 henchmen (about level 7) and about 10 wizards (level 3) for a party of 6 level 12 characters? It would be a tough encounter, no doubt. The PCs level 6 spell slots would be vital and there would be no direct countermagic from the Antagonists - on the other hand the Antagonists would have a better action economy, even if their corresponding spells would be much weaker.

I am thinking setting it in an underground lair in a cave - next to a fast flowing river. Plenty of scope for athletics checks to swim across or use spells/class abilities for the same - or to take the bridge where the main bad guy can hold a chokepoint.

[Tanarii]

Compelled DuelI take it your mooks are generally suicidal? :smallbiggrin:

Of course, if they actually realized what they'd be up against, they probably wouldn't have taken the job in the first place. :smallwink:

[MrStabby]

I take it your mooks are generally suicidal? :smallbiggrin:

Of course, if they actually realized what they'd be up against, they probably wouldn't have taken the job in the first place. :smallwink:

Pretty much. Well suicidal is maybe a bit strong. Killed if they fail their master and with the assumption that after they go down they may stabilise, recover, learn the error of their ways and go back to their loving families. Of course they will flee as well once it is clear the encounter is lost to them.

[Erys]

Would people think this could be a fun basis for an encounter - a powerful melee character (roughly equivalent to a level 16 PC), 4 henchmen (about level 7) and about 10 wizards (level 3) for a party of 6 level 12 characters? It would be a tough encounter, no doubt. The PCs level 6 spell slots would be vital and there would be no direct countermagic from the Antagonists - on the other hand the Antagonists would have a better action economy, even if their corresponding spells would be much weaker.

I am thinking setting it in an underground lair in a cave - next to a fast flowing river. Plenty of scope for athletics checks to swim across or use spells/class abilities for the same - or to take the bridge where the main bad guy can hold a chokepoint.

If you use superior placement with even average tactics, those 10 wizards are going to light up your PC's. I can see it now, 'ok player A, you get hit by... 30 magic missiles.'

That said, it could be a pretty righteous encounter.

If you want your Big Bad warrior to be the center of combat, you could tweak those wizards into clerics who are spamming healing word on their boss while having many similar stall spells available when he doesn't need heals.

[Sigreid]

I take it your mooks are generally suicidal? :smallbiggrin:

Of course, if they actually realized what they'd be up against, they probably wouldn't have taken the job in the first place. :smallwink:

http://www.thethingswesay.com/img/1922.jpg

[8wGremlin]

10 * vHuman 2nd level Warlocks, moderately armoured (AC = 18)
Each armed with an agonising eldritch blast are tough opponents!
They have Scalemail, use a shield and have nothing else in their hand (so can cast)
Tactics are move and blast

[JackPhoenix]

Build them as NPCs, not with player classes. Worried about too low hp? Just increase them and recalculate the CR. Want them to have spells from different class spell lists? Nothing is stopping you. Give them whatever proficiencies fits best. Look at cult fanatics (6HD, 4 level caster at CR2) in MM. Or cult sorcerers (http://www.5esrd.com/gamemastering/monsters-foes/npc/cult-sorcerer) in 5e SRD (pretty much the same thing, but different spells). Or take spellcasting monster of appropriate CR and refluff it.

You're the GM. You shouldn't limit yourself to stuff available to players if it leads to a better game. This is not 3e

[JellyPooga]

Don't worry too much about AoE's ruining the encounter; just make the room big enough and spread 'em out; Wizards and Cultists like standing in big circles and other ceremonial geometric shapes (octagons, pentagons, hexagons and so forth) for summoning, rituals, AGMs...Just make sure you give the impression that they're up to something ritualistic by describing the silver, runescribed lines all over the floor and the tea urn and plate of biscuits in one corner.

I don't know if you've got plot arranged, but perhaps the ritual they're performing is something to do with imbuing the BBEG bruiser dude with demonic power or something...puts a nice time limit on a campaign knowing that if you're quick, the big bruiser warlord you've been sent to kill will just be some guy and not some half-demon monstrosity :smallwink:

I would tend to avoid the "auto-kills" of the likes of Hold Person or Heat Metal; anything that directly takes a character out of a fight or has no save can be frustrating for Players to deal with, especially if it results in them going down with only a single die roll determining their fate. You've got a lot of spellslingers to get a given spell off, even if it takes a couple of attempts, so your spells are going to go off. Use this to proc some spells that you wouldn't normally use as a PC or use them in different ways. For example;

Strange as it sounds, but this might be a scenario where Witch Bolt becomes a useful spell; under normal circumstances it's pretty bad because just moving out of range breaks it. In this scenario, however, you've got a heap of mook-casters; if several of them have it cast on a PC from all different angles, then moving out of range could be a tricky proposition. Especially if the only direction to get out of range is directly towards the melee-focused BBEG. In addition, it would probably blow the campaigns visual SFX budget.

Hex is also one to consider; it's not often you get a chance to Hex the entire party! Some possible fun to be had with Hexing Intelligence and abusing Illusions (it's not a Save to see through an illusion; it's an Int(Investigation) check). See how much the party enjoy dumping Intelligence now...

Ray of Sickness isn't much to look at on paper for a PC; too many things resist or are immune to poison, but unless your PC's are a party of Dwarves, not many PC's are. Poisoned is a nasty little condition and a bunch of low-level spellslingers launching it around instead of Magic Missile (which, while efficient, is boring) could be fun. Same goes for Ray of Enfeeblement; it doesn't usually see a lot of use by PC's, because spell slots are limited and high Con Saves are common, but you might get some use in this scenario because you've got a lot more spell slots to play with, so if the first effect ends...just ping another one at that Barbarian.

Use the likes of Thunderwave and Gust of Wind can be used to move PC's around and into the destructive paths of the BBEG and his lieutenants (as well as away from the low-level casters themselves).

Liberal application of Guiding Bolt (for the more divine minded mook spellslinger) could be lethal in this scenario. Especially if you use the Ready Action to cast it during or between the beefier NPC's turns.

Heroism, Faerie Fire, Enhance Ability, Invisibility, Silence, Flaming Sphere and Moon Beam are all spells I'd recommend for this style of encounter as well.

[MrStabby]

10 * vHuman 2nd level Warlocks, moderately armoured (AC = 18)
Each armed with an agonising eldritch blast are tough opponents!
They have Scalemail, use a shield and have nothing else in their hand (so can cast)
Tactics are move and blast

My problem with this is that it isn't really a bad guy with caster support anymore. It is a bad guy with soldiers - mechanically it is pretty much identical to shooting arrows but with a worse dex save. Not saying it isn't cool - just seems like a bit of a wasted theme for the fight. Also, and maybe I should have made this clearer, I was hoping to avoid attack rolls a bit.


Build them as NPCs, not with player classes. Worried about too low hp? Just increase them and recalculate the CR. Want them to have spells from different class spell lists? Nothing is stopping you. Give them whatever proficiencies fits best. Look at cult fanatics (6HD, 4 level caster at CR2) in MM. Or cult sorcerers (http://www.5esrd.com/gamemastering/monsters-foes/npc/cult-sorcerer) in 5e SRD (pretty much the same thing, but different spells). Or take spellcasting monster of appropriate CR and refluff it.

You're the GM. You shouldn't limit yourself to stuff available to players if it leads to a better game. This is not 3e

This is what I normally do, and I am happy to mix and match (especially with spells - no need for spell effects to even match PHB spells). Where I do have more humanoid opponents though I like to have them resemble classes so the players feel the world is consistent - they could train to be like the enemies they meet. Sometimes this works in reverse and I build a class progression round a NPC idea (which players have sometimes taken levels in). If I can get the effects I want whilst keeping things consistent with what PCs could do i will. I think it is also quite nice that PCs can infer abilities/capabilities: "so these guys are casting spells only available to wizards, it would be logical for them to use their most powerful spells first, their highest spell slot used so far is level 2... so i deduce that a fireball will kill each of them it touches"


If you use superior placement with even average tactics, those 10 wizards are going to light up your PC's. I can see it now, 'ok player A, you get hit by... 30 magic missiles.'

That said, it could be a pretty righteous encounter.

If you want your Big Bad warrior to be the center of combat, you could tweak those wizards into clerics who are spamming healing word on their boss while having many similar stall spells available when he doesn't need heals.

I had somehow missed magic missile. This actually seems about right. Given the fraction of the encounter's XP these mooks would be, making a character unconscious for a turn for all of them seems roughly the right power level. They will die quickly enough so won't be doing that for a second turn. The healing word thing could certainly be fun - not just for keeping enemies up, but for returning a downed badguy to his feet. Just an extra touch of excitement.


Don't worry too much about AoE's ruining the encounter; just make the room big enough and spread 'em out; Wizards and Cultists like standing in big circles and other ceremonial geometric shapes (octagons, pentagons, hexagons and so forth) for summoning, rituals, AGMs...Just make sure you give the impression that they're up to something ritualistic by describing the silver, runescribed lines all over the floor and the tea urn and plate of biscuits in one corner.

I don't know if you've got plot arranged, but perhaps the ritual they're performing is something to do with imbuing the BBEG bruiser dude with demonic power or something...puts a nice time limit on a campaign knowing that if you're quick, the big bruiser warlord you've been sent to kill will just be some guy and not some half-demon monstrosity :smallwink:

I would tend to avoid the "auto-kills" of the likes of Hold Person or Heat Metal; anything that directly takes a character out of a fight or has no save can be frustrating for Players to deal with, especially if it results in them going down with only a single die roll determining their fate. You've got a lot of spellslingers to get a given spell off, even if it takes a couple of attempts, so your spells are going to go off. Use this to proc some spells that you wouldn't normally use as a PC or use them in different ways. For example;

Strange as it sounds, but this might be a scenario where Witch Bolt becomes a useful spell; under normal circumstances it's pretty bad because just moving out of range breaks it. In this scenario, however, you've got a heap of mook-casters; if several of them have it cast on a PC from all different angles, then moving out of range could be a tricky proposition. Especially if the only direction to get out of range is directly towards the melee-focused BBEG. In addition, it would probably blow the campaigns visual SFX budget.

Hex is also one to consider; it's not often you get a chance to Hex the entire party! Some possible fun to be had with Hexing Intelligence and abusing Illusions (it's not a Save to see through an illusion; it's an Int(Investigation) check). See how much the party enjoy dumping Intelligence now...

Ray of Sickness isn't much to look at on paper for a PC; too many things resist or are immune to poison, but unless your PC's are a party of Dwarves, not many PC's are. Poisoned is a nasty little condition and a bunch of low-level spellslingers launching it around instead of Magic Missile (which, while efficient, is boring) could be fun. Same goes for Ray of Enfeeblement; it doesn't usually see a lot of use by PC's, because spell slots are limited and high Con Saves are common, but you might get some use in this scenario because you've got a lot more spell slots to play with, so if the first effect ends...just ping another one at that Barbarian.

Use the likes of Thunderwave and Gust of Wind can be used to move PC's around and into the destructive paths of the BBEG and his lieutenants (as well as away from the low-level casters themselves).

Liberal application of Guiding Bolt (for the more divine minded mook spellslinger) could be lethal in this scenario. Especially if you use the Ready Action to cast it during or between the beefier NPC's turns.

Heroism, Faerie Fire, Enhance Ability, Invisibility, Silence, Flaming Sphere and Moon Beam are all spells I'd recommend for this style of encounter as well.

In rough order - I agree on the not worrying about the AoE thing. Players getting the satisfaction of a powerful move is part of the fun for them and as you say spreading out solves a lot of problems.

Infusing with demonic power? Yeah, sounds like fun. Maybe some other plane to help create a theme - drawing power from the shadowfell or something? Fun times anyway.

Regarding hold person or heat metal - hold person is mean but heat metal seems fair game to me. It is damage and disadvantage, is a long way from an instant kill and can be sorted by one failed concentration save. Hold person seems less fair as you are simultaneously cutting down on the parties ability to force concentration saves (yes disadvantage does the same but with an AC of 13 a level 12 armoured guy should still hit, even with disadvantage.)

Witch bolt? I do see your point, the issue then becomes that they seem a bit too much like martial mooks then, rather than casters. Each turn action being used for damage is OK, but I might not want too many of them. Also, attack rolls are sometimes hard.

Hex + illusions... now you are selling me. This is both cruel and funny and will also make a memorable encounter. Second guessing of what is real and trying to work out what is an illusion from the behaviour of the NPCs is fun.

Both ray of sickness and ray of enfeeblement I considered but was a bit put off by the attack rolls. Style wise they have some good points and can certainly reinforce the mean wizard vibe.

Gust of wind I had given no thought to and thunderwave was only a brief consideration. The idea of using these to manoeuvre others is pretty neat and in keeping with the partial theme of the encounter that the low level casters make the high level beefcakes more scary. I am thinking the martial henchmen can get some whirlwind attacks or something to that kind of positioning works.

Guiding bolt - whilst I had kind of dismissed the attack roll spells, this is pretty damn nasty and also in keeping with the buffing other attacks. Mass guiding bolt is potentially a real issue as one one lands and it gives advantage to the next, which gives advantage to the next and so on. My worry is that this wouldn't be fun for one of the players... on the other hand diamonds are a girls best friend?

Heroism: A tough call on this one. On one hand it is a perfect spell - a nice buff, an incentive to break caster's concentration and I don't see it enough to be bored of it. The downside is that either there is only one caster, which slightly undermines the hordes of wizards theme or I have to keep track of which creatures are buffed by which spell to know what happens when concentration goes. Also tracking temp HP on 7 or 8 creatures is a bit of a bookkeeping pain.

Faerie fire: A great pick. Certainly a good one on the list - the choice of spell may go for the theme of the encounter. Any kind of fey theme or similar could have this work well. It lacks a certain something for a bunch of necromancers. Easy to re-fluff if I need though. I do like that this adds to the terror of the big guy.

Enhance ability: ok... but I just prefer enlarge/reduce

Invisibility: again this is fun, mainly to have the army of casters be unnoticed. I imagine the sense of drama this could bring - also the fear factor as the PCs, used to fighting level 8 to 18 casters, may not assume that the legion of casters are level 3.

Silence: I rejected this at first as i thought it would get in the way of the casters. Now... still not sure but I can see some fun tactical uses. Cast it over the main fighter, who will be engaged with the melee PCs... so the casters are forced to step away from their meatshields. Could work with some more melee to ambush the casters as they sit back.

Flaming Sphere/Moonbeam: well anything with saves is good, area could be nice... 10 moonbeams in an encounter would really set a strong flavour that would be memorable. Between area denaial and direct damage they seem sound. Surrounding the main guy with a ring of death if the PCs try and focus him down may make the engagement a bit trickier. Generally I am a fan of moombeam for character reasons anyway.

I am guessing that if I go for a Fey theme I would have divine bolt, moombeam, faerie fire, spike growth, invisibility

If I go for a more arcane setup then: web, magic missile, enlarge/reduce, burning hands, heat metal


I think there could be some scope to turn this into a fun encounter.

[Vogonjeltz]

A couple of sessions ago I had my players face off against a campaign lieutenant. This guy was a pretty high level caster with a good number of meat minions in the encounter. I have seen this kind of thing a lot - the smart wizard with the dumb bodyguards.

I was thinking it could be fun (if more intensive bookkeeping) to antagonist who is a big tough melee boss, but is supported by low level caster minions.

The problem with this is that often the caster classes are too low on HP to not go down too quickly. One fireball can take out four or five level 3 wizards.

To work around this, whilst still keeping with the theme I wanted to work out what might be the best level 1 and level 2 spells vs a mixed party of adventurers.

My shortlist:
Bane (OK as it is likely to stick and can hit a lot of players. Downside is concentration so it can be stopped and the PCs will go from almost certainly hitting to almost almost certainly hitting)

Burning Hands (damage doesn't usually scale well but doing 3d6 damage to potentially a bunch of PCs isn't bad for a mook and an action. Its a save and not all players dex saves will be any higher than their lvl1 character)

Compelled Duel (not often a great spell but for a low level NPC to potentially take a much higher level PC out of the fight for a turn or so whilst they run away from the rest of the pack to deal with this guy. Bonus points for getting PCs to run over traps or to waste more time due to difficult terrain. Also see spike growth)

Dissonant whispers (Similar function to the duel, but a shorter more intense spell. It allows opportunity attacks and by front loading its effects you don't worry when the caster dies)

Blindness/Deafness (no concentration is good, but given the low level and limited number of spell slots this is less of a benefit)

Arcane lock (maybe can't be used that often, but cutting off one half of the party behind a tough door might be very effective)

Darkness (I include this simply for the surprise element, whilst the antagonist side may be equally hindered the PCs not knowing a) the numbers of bad guys or b) that the main bad guy is in the room may lead to some sub optimal decisions. 15ft radius isnt much though)

Enlarge/Reduce (This can be a pretty hefty buff or nerf for a level 2 spell; having 4 or 5 casters that know it can enable the big melee guy to be a titan of destruction till the support casters get dispatched. Less good as an offensive spell simply because most classes that rely on strength also have good con saves.)

Heat Metal (nothing particularly special about the circumstances just such a good spell that it keeps its value. The (kind of) lack of any saves is a big plus when going up against higher level enemies so I suppose this counts))

Hold Person (Again, just another mean spell. Wisdom save or be out of the fight and be very vulnerable to damage)

Levitate (what would be a great control spell on just about any save but Con; typically poor con saves correlate pretty well with having worthwhile ranged attack options)

Mirror image (worth it if the NPC can pre-buff before the fight, else they are probably better off directly casting a spell. These NPCs are not strategically worth keeping alive)

Spike Growth (the more time the PCs spend trying to dash about and mop up low level mobs hindered by their slow speed the longer bad guys can be pounding on them. A large area of difficult terrain that may not be expected can have a pretty big impact)

Suggestion (OK, this is a bit different but laying out a few on these on the party before the final encounter could produce some chaos. (I was wondering if the recipient would know that the spell was a suggestion spell? Maybe an arcana check at the time the suggestion is made. I was thinking that if the spell was "When you meet evil Baron Von Fellevilo you should begin the encounter by throwing a fireball at him", said several hours in advance then the save would be made when the condition was met, not when the spell was cast)

Water Walk (having a bunch of ranged hostile NPCs on the surface of deep water supporting an encounter may make them a tougher threat to eliminate.)

Web (usual ways the spell is good, plus even a level 12 party can fail a Dex save on their melee guys)


So these are my shortlist spells for my level 3 casters. It looks like the class with the most options is probably Wizard. Even I am not prepared to go with a chorus of divination wizards all spamming portent dice to force failed saves (plus the bookkeeping would be horrific). Possibly Enchantment, with Hypnotic gaze could give some useful stalling abilities to the casters for after they run out of spellslots.


Would people think this could be a fun basis for an encounter - a powerful melee character (roughly equivalent to a level 16 PC), 4 henchmen (about level 7) and about 10 wizards (level 3) for a party of 6 level 12 characters? It would be a tough encounter, no doubt. The PCs level 6 spell slots would be vital and there would be no direct countermagic from the Antagonists - on the other hand the Antagonists would have a better action economy, even if their corresponding spells would be much weaker.

I am thinking setting it in an underground lair in a cave - next to a fast flowing river. Plenty of scope for athletics checks to swim across or use spells/class abilities for the same - or to take the bridge where the main bad guy can hold a chokepoint.

Silence prevents burning hands from being cast; so there's that. You might consider making the guy holding the choke point an Eldritch Knight who uses blade ward (if he's 7th level he'd also get 1 attack) while the minion casters provide fire support.

[smcmike]

Fun question. I like Jellypooga's ideas, as usual. Guiding bolt is great.

One way to make the caster mooks last a bit longer is to have reinforcements join the fight in subsequent rounds.

In the opening round, you can give the players a nice big clump of mooks to fireball, and send a few runners down some back passageways to fetch help The players have to keep the casters from reaching critical mass by doing continual crowd control while also dealing with the Big Bad.


Another, different scenario - an arena fight against a big dumb monster (Rancor, for example), surrounded by weak caster spectators who enjoy messing with the contestants with various spells.

[BW022]

Mr. Stabby,

There is a downside to encounters like that... they have a good likelihood of either becoming trivial (the players let loss with big spells which destroy the low-level casters) or it becomes a time waster (the low-level casters have little effect other than to spend actions on initiatives, saving throws, tracking lots of little spell effects, minor damage to the party, etc.)

If you wish to do this... have the low-level casters pre-buff and have set actions so that you don't have to waste game time making decisions. Consider pre-rolling their initiatives also.

[MrStabby]

Yeah, I agree this can get complicated so I am looking to have a simpler spell list and to do what I can to make it simpler.

Regatding pre-rolled initiative, I would prefer to just have my enemies take 10.

For keeping track of spells - each concentration spell will have a counter and the counter will go on the appropriate enemy to mark who holds it.

I will keep track of spell slots for the encounter as a whole - 2 level 2 and 4 level 1 slots. Each caster will cast level 2 slots first then level 1 slots. If they are incapacitated or miss a turn they lose the slots.

Low level spells are not pointless - the whole point of this was to find the spells that would support an interesting encounter.

Certainly I take your point that there are risks to it slowing down the game and the need to keep things fast paced.

[JellyPooga]

Build them as NPCs, not with player classes.

You're the GM. You shouldn't limit yourself to stuff available to players if it leads to a better game. This is not 3e

This is an important point I missed first time around.

Built as PC's, you're likely to end up with a heap of abilities you might feel like you should be using, but that doesn't necessarily have to be the case. Only give them the abilities you want them to have and will actually use.

Want them to have spells from both the Cleric and Wizard list? Go ahead. Want them to have something like the Eldritch Sight Invocation, but not any other Warlock features? Sure, why not? Want them to have an ability that increases their Save DC when a group of them cast the same spell? Yeah, I don't think there's any precedent for it, but there's no reason you couldn't.

You might want to take a look at the Hag Coven as well, for some inspiration. Perhaps giving them access to higher level spells the more of them there are; individually they're just a bunch of 3rd level NPC spellcasters, but as a group and standing in the "magic circle of power", perhaps their "caster level" goes up by one per member above one, with a shared pool of spell slots. For example;

1 guy has 4xlvl.1 spells and 2xlvl.2 spells
2 guys each have the same as 1 guy, but an additional 2nd level slot between them.
3 guys have an extra 2nd and an extra 3rd lvl slot.
...
10 guys would have access to 5th level spells!

Perhaps something to save for a standalone encounter, because of the extra book keeping involved, but I think it might be fun to describe how the "lines of power seem to diminish as you cut down the mage" and all that jazz. It'd make for an encounter that gets progressively easier the more foes you kill, which in itself could produce some interesting results. I dunno, though, it's just an idea.