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This is another devil that breaks the “morphology” guidelines, but it’s kind of unavoidable this time. After all, they’re associated with Baalzebul, Lord of the Flies.
Each swarm devil is a singular mind controlling the bodies of an entire swarm of acidic fiendish flies. According to the book, they’re formed when an angel is caught lying to its divine master and is torn apart as punishment. The angel’s soul falls to the Hells and the bits of its body become flies. They roam the stagnant pools of Maladomni, the Seventh Hell, eternally starving but unable to die. They can only leave when their master orders them on a mission, which makes them very happy because it allows them to fly free and eat people for the duration. It helps that “eating people” usually is the mission. It’s what swarm devils do best, after all.
Swarm Devils fight by just kinda walking into enemies’ spaces and letting their flies do the work, and by spewing flies at people they can’t easily reach that way. As swarms, they’re hard to fight unless you have area-effect weapons and spells.
These creatures are Large Immortal Magical Beasts (Devil, Swarm) and Level 18 Skirmishers with 174 HP. They have Darkvision and a flight speed of 10 (hover), and 20 acid resistance.
They also have the standard swarm traits on top of that: half damage and immunity to forced movement from melee and ranged attacks; Vulnerable 10 to close and area attacks; a swarm aura; the ability to occupy the same space as other characters; and the ability to squeeze through small openings without slowing down.
Their Swarm Attack aura (1) deals 10 acid damage to enemies caught inside. I think being in the same space as the swarm also counts as being inside the aura. Their basic Slams do no immediate damage, and inflict 10 ongoing acid damage.
They can channel Acidic Curses through their acid, targetting someone taking ongoing acid damage with a ranged attack that deals psychic damage and inflicts a -2 penalty to attacks for a turn. Twice per encounter they can Spew Flies in a Close Burst 3, inflicting ongoing acid damage and forcing targets to grant combat advantage (save ends both).
The “just walking into people” tactic is represented by Swarm Shift (recharge 5+), in which it shifts its speed and deals 15 acid damage to anyone they pass through.
A good tactic for them is to move in, hit a few people with the slam or spew, use Swarm Shift to move out of the scrum and keep flinging curses at anyone who’s melting. Reapply the acid as necessary.
Fly-themed fiends for the Lord of the Flies, nothing unexpected. The acid/psychic damage mechanic is mildly interesting.