Shaitan
About
It does not appear and it does not strike. It suggests. The idea arrives in the middle of your own thinking, in your own voice, indistinguishable from something you worked out yourself, and it is always the thing you were already half inclined toward. By the time anybody realises there is something in the room, the party has spent three days acting on resentments they did not know they were carrying and the damage has all been done by their own hands.
Stats
Save Rolls
Traits
Special Attack
The Whisper: 60 ft, no line of sight required, and it does not reveal its presence. Save Wis Mind, fail save roll 2d10+5 psychic and the target acts on its most private grievance against another creature present on its next turn. Cooldown 2 rounds. The DN should ask that player what the grievance is and use what they say. Bare Hands: bludgeoning, melee 5 ft, +6 to hit, 3d8+5, used only when it has been found, and it would much rather not be.
Special Defense
Not in the Room: while unrevealed it cannot be attacked, targeted or located by any means, and the party will be fighting each other. Named Aloud: a creature that says plainly what is happening - that the thought was not theirs - reveals it to everyone present and it can be attacked normally from that moment. Reduced to 0 HP it goes, and every suggestion it planted stays exactly where it was put.