SkullGreymon is an alternate Ultimate form of Greymon, but its power could be likened to that of a Mega Digimon. The "skull" prefix in his name refers to his body, which is fleshless and entirely composed of bone. A missile lies on his back that is shaped like a shark. SkullGreymon is a destructive and uncontrollable Digimon that results when Greymon is induced to Digivolve incorrectly.
First appearance:
In Digimon Adventure, Tai desperately wanted Greymon to Digivolve further when he gained his Crest of Courage, an object that would help Greymon obtain his next level. When the evil Etemon confined Tai and the other DigiDestined children in a colosseum he forced Tai's Greymon to battle a wild Greymon. Tai purposefully threw himself in front of the evil Greymon to make his partner Digivolve. (The partner Digimon gained new levels primarily when their DigiDestined were put into mortal danger.) This caused Greymon to Dark Digivolve into SkullGreymon,[8] who easily defeated the enemy Greymon. However, he attacked Birdramon (Biyomon's Champion form and the partner of Sora Takenouchi), Garurumon (Gabumon's Champion) and Kabuterimon (Tentomon's Champion) and fled the colosseum. Exhausted, he reverted to Koromon.
This experience shows how the mental state of a DigiDestined can affect the way his/her partner will Digivolve. Each of the DigiDestined in Adventure were bestowed personality traits (in Tai's case, Courage) that they had to display to unlock their Digimon's Ultimate levels. Counter to his trait, Tai had been exhibiting feelings of fear and helplessness during Greymon's battle, which caused Greymon to Digivolve into the wrong Ultimate.
Second appearance:
SkullGreymon makes another appearance in Digimon Adventure 02. When the Digimon Emperor took control of Greymon with a Dark Ring, he tried to force him to Digivolve into MetalGreymon. The result was the corrupt Digivolution that had Greymon Digivolve to SkullGreymon for a second time. Skullgreymon defeated the new DigiDestined with just mere hits, however he managed to defeat one Meramon, some Airdramon and a big group of DarkTyrannomon and destroying a Control Spire with his Dark Shot. The Emperor took him back to his base where he attempted to trigger the correct Digivolution several times as well as find a way to control SkullGreymon (Even if it wasn't MetalGreymon, it was still great power that could achieve results if controlled properly) until he managed to get Greymon to become an evil version of MetalGreymon by using a Dark Spiral (created of 3 Dark Rings joined together).