The Human's Trident [MHA Mermaid Au]

In a world where humans and merfolk coexist, Izuku Midoriya has always wanted to see a mermaid (or merman). His idol and mentor, the marine biologist Toshinori, was the one who discovered the coral merfolk. So naturally, people expect Midoriya to discover something related to merpeople too. And he does.

A mysterious trident washes ashore one night, and as Midoriya attempts to retrieve it he soon finds himself transformed into a merman! The trident speaks to him, telling him the reason he became a merman, his mission. As it turns out, there are other mer-tribes apart from the coral merfolk, and it's Midoriya's job to reunite them all under one rule. But how can a simple human-turned-merman possibly hope to achieve something he didn't even know could happen?