Legend
Saint Mihai is the patron saint of Svartrau's cathedral. According to early chroniclers, he was born a pagan and became a fearless knight who died fighting a terrible monster. His remains were entombed in the crypt beneath the Svartrau Cathedral, and he was venerated as a saint in the centuries that followed.
The official history, as it turns out, is wrong. Mihai was not dead. He was buried alive.
The truth
When Coen enters the cathedral crypt during the quest "A Monk and a Saint," he discovers that Mihai is still alive, or at least, still animate. Mihai awakens and attacks Coen, mistaking him for someone named Tobias. The identity of Tobias and why Mihai expects him has not been explained.
After the fight, Mihai reveals the truth: he is a Dawnwalker, just like Coen. He has been trapped in the crypt for what appears to be centuries, sustained by his hybrid nature. This makes him the second known Dawnwalker in the game's lore and confirms that Coen's condition is not entirely unique. It has happened before, under different circumstances.
The Khoboldin sword
After the fight, Mihai gives Coen a task: find the scattered fragments of the Khoboldin sword and reforge it. The Khoboldin sword appears to be a legendary weapon of some significance. The quest to locate its fragments and reassemble it is one of the game's major questlines.
Implications
Mihai's existence raises questions about the Dawnwalker condition. If Mihai was turned centuries ago, it means the silver-poisoning-plus-vampire-turning combination has occurred before, or there are other ways to produce a Dawnwalker. The fact that he survived burial for centuries also demonstrates that Dawnwalkers age at a dramatically reduced rate, consistent with what the developers have said about Coen's own slowed aging.