Wynter (The Men Who Sold the World)

You may be looking for the Time Lord Castellan.

Mr Wynter was a hired assassin who worked for the American government. He claimed to have held the brains of dead presidents in his hands.

When the British government sold the American government alien weapons salvaged from the ruins of the Torchwood Hub, a CIA special ops team used as middle men went rogue and decided to keep the weapons for themselves. The group, headed by Cotter Gleason, planned on killing the American president. Wynter was hired to prevent this from happening.

Wynter used an Ytraxorian Reality Rifle to travel through time. He rewrote history so that Rex Matheson and Glen Shaeffer prevented the assassination of the president and the destruction of the White House. Mr Wynter destroyed all the weapons except for the Ytraxorian rifle, which he kept. Mr Wynter called Matheson, informing him that he would leave him alive because he admired the spark of idealism he saw in Matheson; but that he would have to kill him if he ever saw him again. Mr Wynter then pulled a few strings to restore Matheson's reputation in the CIA, allowing him to continue as a CIA operative. (PROSE: The Men Who Sold the World)