Play Auchenai Soulpriest, Auchenai Phantasm, or Embrace the Shadow
Cast a spell that restores health
The spell should have the healing numbers converted into damage and then amplified, but the last part isn’t applied. For example, Regenerate (restore 3 health) with a card that grants spell damage +1 should deal 4 damage, but deals 3 instead.
I believe that this used to work differently, but has been changed because there were too many inconsistencies with the old method.
How it now works is that spell damage is applied to spells that cause damage, and after that, all the various “turn healing into damage” effects are applied.