This cell looks like it is the same size as the others, but it begins to shift and change.
This effect is an illusion and a very powerful one. This illusion makes the room look larger on the inside than it is. Additionally, there is an effect that will confuse anyone inside. The room's illusion is not related to the creature inside and a save vs. Magic will reveal the room for what it is.
The inhabitant of this room takes advantage of this illusion. Inside this room is a Will-o-the-wisp.
AC 3 [17], HD 5+1 (22 hp), Att 1, life drain, THAC0 15 [+4], MV 90’ (30’), SV D11 W11 P12 B13 S15, ML 12, AL Chaotic, XP 575, NA 1 (1d4), TT see below.
This is not the Will-o-the-Wisp normally encountered. This is an undead creature. While it has high HD it is a single creature and has low hp. It can be turned by a cleric as a 5HD creature. The creature can only be hit by magic. Its low AC is due to its size, its erratic movement, and its magical protection.
The wisp leads characters into this room and uses the illusion to keep victims confused and scared. It then feeds on the life energies of those trapped. It can't drain the victims directly and has to wait till they start to die of other causes.
The Wisp does not collect treasure, but there is a lot material left from former victims. The only way to find it though is make the save vs. spells to stop the illusions.
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I am reworking Will-O-the-Wisps for my Basic Bestiary. This is a rough draft.