The Heart of Darkness appears as a slender, white crystal. It constantly floats whenever it is not bound by another substance and can cover the user in powerful "Dark Goo".
Meen was testing it in his laboratory when it started to move around of its own accord. Then it zapped his PAK with dark energy and entered it. Now he has the ability to enter "Dark Mode", where he gains all sorts of dark powers, like the ability to shoot the Dark Beam from his PowerSuit's Arm Cannon, or the ability to enter the Nightmare Dimension without it leeching the life from him slowly and corrupting him. He gains a general invulnerability to any Dark-based dimension or Life-form, except the mirror copies of himself in other dimensions.