TwistedHusk

Nightmare takes his own darkness, and imbues it into a twisted husk, which is summoned when the leftover energy of colle or void is spat out of the crystals.

one such occurrence was when nightmare first took the yellow crystal. A shadow loomed over the room, and bursts of colour began to emanate from the yellow crystal.

Nightmare staggered back, snatching the yellow crystal from the forming tornado of colour. The tornado opened like an envelope to reveal the monster inside.

He and a scared nightmare fought for a while until nightmare manages to capture it, to use it as a test subject.