Magog

Magog was the most fearsome and powerful of the Malevilus that were worshipped as gods by an alternate Roman Empire. He took over the body of Adolphus Caesar's mother Juno, and planned to do the same to Adolphus when he succeeded the throne. The Doctor tricked Magog into being imprisoned in an empty dimension. (DWM: The Iron Legion)

Ace briefly encountered Magog, still trapped in the Doctor's TARDIS and pleading for release. (DWM: The Mark of Mandragora)

Other information
Magog was named as one of the Great Old Ones. It is unclear if the two entities were one and the same. (PDA: Divided Loyalties)