Aibou wa Slime!? ~Saikyou no Aibou o Eta Boku ga Saikyou no Mahou o Tsukatte Nariagaru~

Main character Zack has hit rock bottom.

His family hates him, and his fiancee called off his engagement.
A black object appears before him. It turns out to be Sura, a powerful black slime that speaks human!

A great adventure, with the strongest slime! They're aiming to rule the continent!?