Okay I'm interested. But I got a few ideas to run by you so I know they are allowed (or which ones are and which ones aren't)

So my first idea was to make a none-organic life form. They would look humanoid to a point though would mostly form around a central crystal emitting a strange kind of energy (magic). They aren't completely made out of energy, so they can still be killed. Though 'destroyed' might be a better term.

Another idea is pretty much a completely synthetic life form. AKA robots. Though they wouldn't be after killing every organic civilization out there. If you're familiar with Mass Effect, I'd make them like the Geth.