@Esperantos I think this is how it goes:
1) Beelzebub - "Dark/Divine" ability, double-edged sword, consumes both the target and the user (Ryuuto) at the same time.
2) Beelzebub gets activated, God Eater kicks in, "consumes/seals" all Divine-based spells and abilities targeted at the user into their body, letting them be able to use Beelzebub immediately again.
3) God Eater creates excess energy that the user's body can't handle when sealing, would normally destroy it outright
4) Dragon's King ring takes the energy and uses it back on God Eater to keep it balance with Beelzebub.
TL;DR: Basically Ryuuto has made a magical black hole that uses its targets for fuel, and he gets to use the target's abilities as a bonus.