I'm marking down the Matryomin as the creepiest instrument I have ever seen. The Matryomin is a combination of a nesting doll and a theremin created by Masami Takeuchi.
I have never heard a theremin play until I stumbled across the Matryomin. The music produced is definitely different and it is incredibly cool to see an entire orchestra playing Matryomins to the tune of the Star Trek theme. Only 3,000 Matryomins were ever made, with a few being up for auction on sites like eBay to the tune of $500.