I need a curse that physically alters those who are afflicted, but I didn't want to do Lycanthropy (Don't get me wrong, I love it) cause it seemed generic. I still wanted it to turn them into some kind of monster or animal. Also not sure if I should have them lose most consciousness or be able to play normally still.

Grant them a curse that you tell them turns them into the perfect being. Then slowly make them more of a crab over time, slowly lowering Charisma and perhaps lowering their speed and eventually removing their ability to speak common. If it needs compensation, give them a swim or burrow speed equal to their original speed.