Lord Annihilator, otherwise known as the Nightmare King and the Lord of Darkness, is a member of Lord Dominator's Species who has control over Nightmarium, a material that is considered "evil as a substance."
His real name was Alex, and he was born on the same planet as Dominator. The two grew up together and became lifelong friends. Alex started out as a good person, holding doors open for people, helping them out, and being a lot like Wander, actually. But only Dee was his friend. Everyone else was mean to him and laughed at him because of his dark powers.
Where they came from, people were born with power over certain elements. Dominator, or Dee, had control over Volcanium X (Lava and fire), while Alex controlled Nightmarium (Shadows, Darkness and people's fears). People were fearful of Nightmare-Wielders, since they were so rare and were usually evil due to the evil nature of their power. Dee and Alex became friends because they were both rejected and hated by society. They grew up as misfits, running around doing whatever they want, when they wanted. As they got older, they even started dating for a while.
But, since we all know Dominator is simply an evil person, she broke up with Alex just to be amused by his resulting misery. When questioned by her old friend as to why, she simply said he wasn't "bad enough", that he was "too nice".
Alex was heartbroken by this, and decided from that point onward to be evil enough for her. Taking her words way too literally, he started by destroying their home planet, building an army of Nightmare-powered doombots, and destroying planet after planet, only to rebuild those planets as Nightmare Worlds, planets that constantly radiated the same brown darkness energy released by the Black Cube. Dominator built her own army and the two faced off. The battle was long and hard, but it ended with Annihilator being cast into a black hole, along with his Ship and army. Dominator then fled her home Galaxy, not knowing how to destroy the Nightmare Worlds Annihilator had created.
Years later, after Dominator's defeat at the hands of Wander, Sylvia, and the other citizens of the Galaxy, the Nightmare King returned, seeking to capture and torment Dominator for his own pleasure, and to plunge the Galaxy into an eternal Darkness.