Saint Mihai is a Dawnwalker who attacks Coen when he is freed from his tomb. He is a fast, pure sword duel, and beating him starts the quest to reforge one of the best swords in the game.
Learn the St. Mihai symbol from the cathedral murals or books, then clear the leaves off the marked grave and drop in. In the crypt's middle room, use Focus to spot a broken brick. Inspecting it costs an hour. Lever the gap open and kick down the wall to free him.
There is no gimmick phase. He is fast, closes distance quickly and chains his attacks. Dodge skull-marked strikes and block or parry shield-marked ones, then counter. Dodge through his swings rather than backing off, and punish the recovery after his lunges.
Villagers entombed him alive when they found out what he was. He lost his mind there.
He reveals he is a Dawnwalker and mistakes Coen for Tobias at first. He tells you the Kobolds hold the shards of his sword and asks you to reforge it, which starts Forge It Anew. He dies soon after.