Heart Ablaze grants Karlach Immunity to Charmed effects.
Infernal iron has anchored the war machine that is Karlach's heart.
She can touch others without burning them, and can no longer be Charmed.
Other ways to learn:
Infernal Iron to Dammon in the Last Light Inn, as part of the quest The Hellion's Heart.
Infernal Iron to Dammon in the Last Light Inn, as part of the quest The Hellion's Heart.