Oberon

Oberon was the king of the Sidhe. He represented chaos, whereas his queen Titania represented order.

During World War II, he helped both sides develop tanks which could phase into higher dimensions. As he was chaos personified, he was not held responsible for his actions at first.

Oberon travelled back in time and had degaussing equipment installed on the USS Eldridge, allowing him to be aboard the ship. The Eighth Doctor discovered this when he landed there with Fitz Kreiner and Sam Jones. The Doctor realised that Oberon was now pushing his limits, to see how much chaos he could get away with causing.

However, Sam was really Oberon in disguise. He tried to kill the Doctor, who was forced to deactivated the degaussing equipment and trap Oberon on the misphased ship. When the ship later appeared in the rift between Earth and the Sidhe realm, the ship was destroyed and Oberon killed. (PROSE: Autumn Mist)