Fusing demons together is something that is unique to the Shin Megami Tensei games from Atlus. Those that have been around the RPG block a few times see the words 'demon fusion' and think about how they could get rid of those lesser, more common demons to make something cooler, and with better abilities. Everyone else would think it sounds like crazy talk. Everyone else thinks we're all devil worshipers that drink goats blood and gnaw on crow's feet.
Demon fusion is crazy, but it's also cool and useful, and I'm glad you can do it in Devil Survivor Overclocked, an upcoming 3DS RPG. If mixing beasts together doesn't satisfy your new demon craving know that you can also hit the auction and try to get one at a good price. This trailer does a way better job of explaining how it works, so just watch that instead. I'll see you guys later. I've got to go make my daily sacrifice now.