I have an Eternal and a Shinigami and I want to combine them to make a Abaddon, but I've never combined legnedaries so I don't know how to do this.

Help, please d:

If I'm not mistaken, you have to go to Asmodeus Genetech Lab situated in the Darkside. Just select your Eternal and Shinigami, combine them, and you should get your Abaddon. :)