2. buying spells in the guild allow slaves to learn it.
Acid Spit and Mind Blast are the only two dmg spells in the game if I remember right.
but you are right, it may be a bug.
My slaves will NEVER learn those spells. I mean, have you seen how many skillpoints it costs to teach just one of them? "Sorry guys, no magic for you!" F*ck the guild and its fees! :P
I've only ever taught heal, so that my slaves can be damage sponges and keep me alive in the final battle.