With the hearts or minds of his three lieutenants in hand, you must travel across the wastes and walk the forbidden plains - into the heart of the barren garden. It's here where his power is strongest, but he will show himself nowhere else. He has no reason to step down from his throne.
Yet in this place, if you call him with the voices of his lessers, then he will come. With all the might and hatred of the violet poison from which he was born, he will answer your call:
The King of Shadows. The King of Death.
Do not summon him recklessly. Before you challenge him, you'll need to prepare. As it happens, this is one thing I can help you with.