The following units can learn dark magic:
liches, dark priests, villains, and also the sniper. Can ninjas also learn it?
Yes. Advanced monks or ninjas can learn dark or light magic. Up to the expert level, I think. But I haven't done it myself – they have plenty of fun without magic.
Hmm... I think it's not the sniper who can learn dark magic, but the bounty hunter (dark ranger). Or am I mistaken? :confused:
