In the Middle Ages in Europe, smoke was everywhere and the plague was rampant. A patient with "habitual time travel syndrome" unfortunately traveled here.
This man has great supernatural powers and is good at committing suicide. Even if he cannot distinguish between medicine and surgery, he still dares to teach the European people medical knowledge that is difficult to distinguish between true and false. Together with a priest, he pretends to be a ghost and saves the common people.
And then people in Europe really believed it - but that's not the point.
The point is that this man has experienced feuds, heresy trials, conspiracies and plagues, the fourteenth century and plagues, and other things and plagues. And then until he found himself with a real angel who fell to the mortal world, he didn't know that what he was about to experience was a love more suffocating than death.