The Brain.
It doesn't move, it doesn't see, and it can't even smell you. But what it can do is tell every single monster in the room your position and show them the easiest way to get to you. Created by the Empire to serve as a defense mechanism, much like the Evil Eye, this almost lifeless pile of flesh can easily turn into your worst nightmare. This monster will change the movement patterns of every other monster in the room, leaving you to figure out how to get rid of it before it creates too much disaster.
For the most part, architects tend to hide the brain in a room, either forcing you to keep it alive or making it the last monster accessible to kill. This means that you'll have to adapt quickly and be able to compensate for all of the monsters that will be coming at you much faster than normal. The best thing a Smitemaster can do is to take their time, plan their attack, and make sure that it will work. When a brain is combined with roach queen or mineral mothers, then the swarm that they produce can get even uglier when they have this large piece of pink tissue on their side, so watch your steps.
Detailed Behaviour Analysis
Brains don't move, but their presence in a room affects all monsters in it (excluding guards and slayers). They generate a path map for each monster recursively by calculating the distance from the player in terms of the minimum number of moves it would take to reach him. When calculating this distance, the brain treats as obstacles any room objects that a roach could not step onto ("brain-visible obstacles".) It does not take into account the location of enemies or other "brain-invisible obstacles". If a brain cannot find a path from a monster to the player, the monster acts as unbrained. Lastly, if a brain
is within "smelling range" of the player when he is invisible (whether due to potion or shallow water), all monsters in the room will act as if brained, even if he is outside of their "smelling range".
Brain-visible objects include walls, scrolls, bombs, arrows (regardless of direction), orbs, serpent bodies, tarstuff and mirrors. Whether water or pits are brain-visible depends on whether a monster can normally traverse them.
Brain-invisible objects include other monsters, serpent heads and orthogonal squares.