Who has enough powerful magic.

A couple of questions: how do you manage to level up Darkness skills with the Dark Elves? After all, their second school is Summoning. And even if you take a hero with initial Darkness magic, where do you get the spells? In the Dark Elf guild, there are only low-level spells, and not many of them. The same goes for the Sylvan faction. Their main school is Light, and the second is Chaos.