What if you don't have Illithid that are evolved Aboleth, but Aboleth that are devolved Illithid?
The dying stars in the far future emit strange radiations, that turn Illithid tadpoles into Aboleth. While the Illithid slay such abominions quickly, some of them escape and manage to travel to the...
With kills the caster you mean
1. He appears and kills the caster with no save
2. He appears, is killed by the caster in a couple of rounds and his possession are added to those of the caster?
:]
You should, because players are inherently stupid.
(Any DM who played too could confirm this: he was very smart when he was DM, but made a lot of silly mistakes when he was a player)