Gristlegrim's Monsters Invisible Stalkers

They are invisible, they stalk, they slay with either great invisible claws or invisible blades. Fighting them is extremely difficult because they cannot be seen. One may assume they are humanoid in shape because of the mocking laughter they emit when they strike. All players' weapon rolls are halved when fighting Invisible Stalkers. They can be rendered visible for a few seconds at a time by an Oh-there-it-is spell--if that happens, the players see something rather like a purple gorilla with a machete, but without hair, and with a far more demoniacal face. Luckily for any delvers who meet them, Stalkers only appear one at a time. (Purple is the magical effect caused by the spell--not the natural color of invisible stalkers.)

There are no low-level Invisible Stalkers. They start at Level 4. They are implacable and will keep fighting until dead, or all players have been slain. Even dead, they remain invisible.

Level 4: Monster rating 80.
Level 5: Monster rating 160
Level 6: Monster rating 240

They may come in higher levels, but only up to level 6 inside Gristlegrim.