Kill Doedre Darktongue.
Doedre Darktongue is a unique monster in The Harvest. The Harvest is the final area of Act 4. It contains a waypoint and is connected to The Belly of the Beast Level 2. Its sub-area The Black Core is dedicated to the fight with Malachai, The Nightmare and can be accessed once the three other bosses have been defeated.
She uses a skill that throws red orb projectiles. Her area contains three partially overlapping circles that keep the player afflicted by a curse (if he stands in more circles, he is inflicted with multiple curses):
She will also consume curses inflicted by the player, removing its effects and gaining Unholy Might.
She should be rather easy if you stay out of her projectile.
Malachai’s Lungs is acquired after defeating her.