I enjoy a lot of mosnters here and some have mentioned another favorite. The Astral Stalker. When posting about planar campaigns, I was thinking of having the Astral Stalker be like Predator where they gather dozens of adventuerers, the PC's among them, and let them lose in the Astral Plane and hunt them down. PC's slowly gather strength, watching NPC's get cut down and off, meeting 3rd party denizens that interfer with the hunt, until they gain enough power to fight back and end the hunt for all time! Now reading over Phil's Dozen Demiplanes to see if I can work that angle in there somehow...