A demon lord killed on his/her/its native plane is officially dead. If killed on a different plane, then he/she/it is reborn on his/her/its native plane and is forbidden to leave for a long number of years. Right?
Wat Dagon is described as a pocket dimension, not really Demogorgon's home turf. Being killed there (where the adventure assumes you fight and kill him) is not permanent. Right?
Anyone else come across this problem?