Complex sentence: He is an extra-dimensional magical entity (possibly a demon) which takes vengeance on its enemies, and was invited by Dweller-in-Darkness to join the Fear Lords, but he betrayed them to Doctor Strange.
Split into simpler sentences: He is an extra-dimensional magical entity (possibly a demon) which takes vengeance on its enemies. He was invited by Dweller-in-Darkness to join the Fear Lords, but he betrayed them to Doctor Strange.