Dark Evolution (暗黒進化 Ankoku Shinka, ダーク進化 Dāku Shinka) is a corrupt form of normal evolution.
When Taichi aggressively forced Agumon to try to super evolve in "Dark Evolution! Skull Greymon" with disregard to Agumon and the others' feelings Agumon evolved into Skull Greymon. In this form Agumon lost all sense of himself and attacked anything and anyone nearby.
Digimon Adventure tri.
In Coexistence, Hikari becomes deeply depressed due to her brother's disappearance. As a form to fully release Meicoomon's destructive power, the Mysterious Man converts Hikari's bad feelings into viral data, forcing Nyaromon to dark evolve into Ofanimon: Falldown Mode, that later fuses with Raguelmon into Ordinemon.
A dark evolution occurs when Yagami Taichi gets swallowed by Done Devimon while Metal Greymon: Alterous Mode watches. He was completely succumbed by the miasma which his hate and anger felt by the latter pushes him to dark evolve into Mugendramon.