Darth Rueven [Roo-vun] is a cold and cunning Sith. Brave and fearless, this Sith causes havoc - for the Jedi. He has killed many Jedi in his time. He is a feared Sith and there are many ghastly stories about him.
He is often seen fighting alongside his brother, a Bounty Hunter called Choken who often helps the Sith Order. Choken wears a devilish mask with skull teeth.
Rueven and Chokens' species is not known. They are mysterious and anyone who has encountered them (and lived) has said they are ruthless and cunning.
Just for kicks, his brother Choken.
A closer look at his head/mask.