Dark Corvo is an optional boss in Story Mode in a "secret stage" and a final boss in Classic Mode. He must be unlocked by clearing Story Mode. His battle theme in Story Mode is a remake of "The Encounter" from the Kingdom Hearts series, but in Classic Mode, it's Brawl's Master Hand's version of Final Destination. He's Dark Raven's right-hand crow and a powerful crow wizard (just like his master). He can be fought by anyone in Classic Mode. He attacks by making copies of some defeated bosses (Regigigas, Hewdraw, King K. Rool, etc.) and himself. Once defeated, he lays on the ground defeated and sucks back into the Shadow World. He is voiced by Peter Berkrot, the same man who voices I. M. Meen.
Here's the list of illusions he can create.