The Malevilus were a malevolent humanoid race, described as “a form of anti-life that feeds off death.” they had bat-like wings, horned heads, a general demonic appearance and strange powers.
On an alternate Earth, five Malevilus named Babiyon, Abiss, Epok, Nekros and the most powerful, Magog, took on the form of Roman gods and supplied the Roman army with advanced technology. The Romans, in return, offered up prisoners to them for sacrifice. The Malevilus set up puppet Emperors in their stead and oversaw the expansion of the Roman Empire throughout Mutter’s Spiral.
The Fourth Doctor ended their rule by televising the image of Magog and showing the Roman people the true faces of their “gods”, inciting them into open revolt. Magog was trapped in an empty dimension within the Doctor’s TARDIS, and the rest of the Malevilus tried to escape in their spacecraft, which exploded on take-off, killing them. (Doctor Who and the Iron Legion)