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.