This ability is obsolete. It was removed in patch 0.13+109.
Soulstrike was an active ability that could be learned by any hero.
(Active) Hero attacks with Potency instead of Bonus Damage; her attack is mitigated by warding instead of armor.
The Soulstrike action is added to the hero's ability bar beside the standard Strike or Shoot actions. It works with both melee and ranged weapons, allowing the hero to choose whether to make a physical or magical attack depending whether the target has less armor or warding.
The Staff is a particularly effective weapon with Soulstrike.