Chaos magic. Is it necessary?

Doesn't work... Actually, I would choose Summoning instead of Chaos, if this magic was a core skill for the Sylvan faction...

Yes, I completely agree. Although they are not as weak as the Elven faction (after all, trees and unicorns are quite strong), summoning magic is necessary. It will be versatile for them, providing both defense and offense, and if you take Fire Warriors, you don't even need to upgrade Chaos magic at all. This is the advantage of summoning magic.