Ambition has to be the number one thing. Look at any good dark mage protagonist, from the classics to webnovels, and that drive for power, knowledge, or just survival is the engine. But it can't be ambition alone, or they're just a cardboard villain. The ones that stick with me are always the ones whose ambition is tangled up with something painfully human. Like a scholar's obsessive curiosity that tips over the edge into forbidden lore, or a survivor's desperation that makes any cost seem acceptable.
Then there's this specific flavor of charisma that borders on menace. They're persuasive, but it's a cold, logical persuasion that can be more terrifying than a threat. They make you understand why someone might follow them into the dark, even when you know it's wrong. That detachment from conventional morality is key, but the best ones still have one fraying thread connecting them to something else—a single person they'd protect, a memory of who they were, or even just a quiet, aesthetic appreciation for the beauty of their own destructive magic. Without that thread, they're just a force of nature, and honestly, a bit boring to follow for a whole book.