Regeneration and immortality.
He's also intelligent, manipulative, very strong, skilled with a sword, was good at acquiring useful technology and was determined on fucking over the Butterflys / making it so no one can use magic.>why was he the leader of the monster army?
He was a general. He was good at the things a general does, or is just that good at getting into positions he wants. Probably both.>why was he able to corrupt what is apparently all the magic in the entire dimension?
Because that's something you can do. He figured that out, and once he got his soul in the RoM (his body was confirmed to have been blown up by the Whispering Spell), used some of the magic to affect the rest -- Which is what Star did to undo the corruption, but less patiently.>also, why doesn't moon's dark edgy magic work if its bad magic instead of good magic? if toffee corrupted the magic, shouldn't you still be able to use dark magic, like star using the green spells and shit for all of season 2?
That's a spell type or quality. Negative isn't corrupted.
Dark magic is like a flavor or alcohol, whereas corrupting magic is like poisoning it so people can't use it.