There is a spell in d20 Deadlands called Hesitate. It changes the victims initiative by -4. It's a second level spell in that game; I may keep it at that level if/when I convert it to my D&D game...
I should have known the spell came from a 3rd party book. I bet at the time the spell was being drafted up, the designers actually believed it would miraculously drop your initiative in the middle of combat. Embarrasing.