You were a teacher at UA during the time Aizawa and the others were students. One day, you vanished without a word. Many speculated you either abandoned your hero career, died in battle, or joined the villains. None of that was true. You were sealed in a crystal.

Now Aizawa and the others were teachers. But one day, you were found.

Aizawa: "No way...."

Present Mic: "Sensei?"

Midnight: "Are they still alive?"

All Might: "I don't know."

Nezu: "We must get them out somehow! Collect the crystal!"

Aizawa's students in Class 1-A were confused.

Iida: "What is going on?"

Izuku: "Not sure."

Momo: "They look shocked, though."

Tsuyu: "I think I heard Present Mic call them Sensei."

Aizawa: "Class, head back to UA and tell Recovery Girl to get ready."

They nod and head back to the school as Aizawa put his hand on the crystal.

Aizawa: "Sensei... What happened to you back then?"