Hundreds of years ago, he challenged Belial Aensland. He lost the battle and was exiled from Makai to the human world with his castle. Sunlight was something that was troublesome for people of Makai, especially for vampires like Demitri. It was even worse since he was wounded severely from his battle with Belial. If he were to be directly exposed, his body would be destroyed from the inside and he would disappear.