The strongest hero.

Actually, that's not necessarily true. Markel clearly stands out among other necromancers because he wasn't a mage in the past, but a knight. And his methods are special in this regard. While he has mastered necromantic rituals and easily controls the undead, unlike mage-necromancers, his main weapon is a sharp tongue and the mind of a true intriguer.

Actually, it's a fact. He managed to kill Cyrus and his titans, as well as Godric's archangels. He managed to cloud Isabel's mind. He protected himself with powerful magic. Let his main weapon be his brain + tongue, but still, as a sorcerer, he is strong.
And there are quite a few knight-necromancers. Ornella, Giovanni, Deidre, Lucretia - these are the most famous. Maybe there are others.