So this came up in a game last night. Hastur was the ancient one making all cultists flying creatures. One of the players cast Denying the Ancient one which takes away the special bonuses provided the worshipers of the ancient ones and at the time there were cultists in the sky. We played this as the cultists couldn't leave the sky while the spell was in effect but it brought on many jokes about cultists falling out of the sky to their deaths.
I wanted to post this to make sure we played that correctly and because it was slightly amusing.