The Seep.

To call these creatures "Simple vapors living inside of walls" makes them seem less deadly then they actually are. Although the Empire has just recently been successful in developing a prototype and are only testing them in selective dungeons, they have already created panic and fright inside of the Guild's halls. Before their existance, a Smitemaster could always feel a bit safer when his back was at the wall. Now, they have to watch out for their presense, lest they become a victim to the poisonous gas that they are.

If a door is opened while seep are on it, then they will die. They can also be removed by a strike with a standard issue sword. Although the creature's contents are complicated, the actual movement is very simple (see below). The only problem is getting them to reach the edge of a wall or a door so, like roaches and most other dungeon creatures, you will have to be careful where you lead seep to.

And in case you wondered, seep is used for both singular and plural useage. In other words, you could have a seep in the room, or you could have many seep in the room.

Detailed Behaviour Analysis

The Seep can only travel through walls and closed doors. They can jump out of the walls, only if their move is to kill the player, or stalwart/soldier. Doing this kills them too. This also allows them to step on a tile containing a potion.

Their movement algorithm is the same as the roach's, except when the path they wish to take is blocked diagonally. Here, rather than picking a vertical choice over a horizontal, the seep will pick the one which reduces the longest orthogonal distance to the player. E.g. It will move west if the player is five squares west and four north, and vice versa. If these are the same, then the seep go back to having vertical preference.

Behaviour when brained
Seep use the same algorithm for picking a tile to move into as roaches. The difference is, if blocked by a brain-invisible object, they revert to unbrained behaviour.