As much as I want to say aboleth, I am going to have to go with the secret eater from Bastion's Minions. They are clerics that erase 3 types of memories- the knowledge they protect (or keep from others), their existance with those they encounter and what they look like. The last is may seem strange, but all of them wear masks and they consume that knowledge first whenever they encounter someone who has defeated one. The sociopolitical ramifications of their existance in any nation can be huge. What if arcane magic, a certain god or even the outside world is the knowledge they keep from others? I like them as a start to a campaign where the party learns the truth with other people and watch as the others have their memories erased (or possibly killed and everyone they knew have their memories altered).
The seeder, also from Minions, is also cool, but limited in combat situations- they convert males of other species so that all of their offspring are seeders. I like the biological aspect and the long time that a PC has to learn that he has been changed. I use them as population control for some regions and treated as demons in others (where the males changed are burned for carrying the demon seed).