Evoker I/II/III are collapsed into one icon on the unit detail screen, and only show the tooltip for Evoker III. This makes it particularly hard to calculate total spell damage; I think they stack, but I don't remember the bonus from I and II.
While we are here, why do I have to calculate it at all? Spell damage multiplier should be listed among the stats on the right, next to spell resist and mastery.
