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

Main character Zack has hit rock bottom. His family hates him, and his fiancée called off his engagement. A black object appears before him. It turns out to be Sura, a powerful black slime that speaks human!