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This is another one of those monsters that’s all about blurring the lines between fiends. These monsters are devils, but they break the design rule that says devils have a human-like appearance.
Slime Devils are amorphous oozes, though unlike most “true” oozes they’re sapient and rather smart. They probably originated in some past edition as well, because they’re the second devil said to be formed from the souls of dead sinners (which is not a 4e thing). They arise from the mixture of the toxic mud in the swamps of Minauros with the hatred that seeps from the souls of corrupt inquisitors who are stuck there. Their abilities make them adept interrogators and torturers, since they can read the minds of people they envelop and inflict intense pain on them.
They’re often summoned to act in this capacity by villains in the world, so they’ll be found alongside those villains’ guards and jailers. In Hell, they travel with burlier devils, and with those whose controller-y powers can hold enemies down to be engulfed.
A Slime Devil is a Medium Immortal Humanoid (Devil, Ooze), and a Level 16 Lurker with 123 HP. The lore does say their base form is vaguely humanoid, but still I’d probably classify them as Magical Beasts instead. Either way, it doesn’t change their other stats.
They have Darkvision, ground and swim speeds of 6, and Resist Acid 20 instead of the usual devil fire resistance. Their Mercurial Body trait allows them to ignore difficult terrain and not provoke opportunity attacks when moving. They’re still vulnerable to a Fighter’s Combat Challenge or other similar abilities, but non-defenders can’t hold them in place.
Their basic attack is a Caustic Slam that targets Fortitude and does acid damage. They can also stretch into Acidic Tendrils that attack up to three creatures at Reach 3, deal immediate and ongoing acid damage, and pull them 2 squares.
The slime devil’s main trick is Diabolical Engulfment, which targets reflex and grabs on a hit. Grabbed targets are dazed and take ongoing 10 acid damage while the grab lasts. The creature occupies the same space as the victim, and pulls the victim along when it moves. Attacks that hit the slime devil deal half damage to it and half damage to the victim.
As a minor action (1/round), the slime devil can use Scour the Mind on its grabbed victim. This targets Will. On a hit, the devil poses a question to the victim, who must choose between answering truthfully and taking psychic damage. The amount of damage is the same as a basic attack, which is a lot for a minor-action power. The creature can do this and still use its basic attack to fight off the victim’s friends.
Lore-wise, kinda meh. I’m not a fan of bringing the old “fiends come from the souls of sinners” cliches back into D&D. They’re more interesting mechanically, because I love attack powers that force PCs to choose between two bad outcomes.