History
When the Defenders traveled through the Dark Dimension so they could launch an attack on Apocalypse's fortress, they were confronted by Dormammu. Brother Voodoo offered to stay behind and hold off Dormammu while the other Defenders fled. However, Voodoo's teammate Nathan Summers betrayed him by taking his Eye of Agamotto, leaving him defenseless against Dormammu and leading to his death.
Dormammu later appeared on Earth as Captain America and Wolverine were fighting and announced his intent to conquer their world.[1]