Thorgan was a Sulumian.
Around 317,000 AD, humans encountered the Sulumians in their domain in the eighth dimension. To prevent this from happening, Thorgan travelled back in time to divert Earth's history.
The Eighth Doctor crossed paths with him in 13th century Pisa, where Thorgan was trying to kill Fibonacci before the Italian had written the Liber quadratorum. Swearing revenge after his plot was stopped, he met the Doctor again in 40 BC, by which time the Time Lord was travelling with Fitz and Trix. Thorgan was going to blow up Octavian's villa with a strontium grenade, which would have led to the Treaty of Brundusium not being signed. The Doctor had found the grenade earlier and planted it on Thorgan without the Sulumian noticing. Thus when he detonated the grenade, Thorgan blew himself up instead. (PROSE: The Gallifrey Chronicles)