Sil is a representative for the Galatron Mining Corporation. He comes from the planet Thoros Beta which is the twin of Thoros Alpha. Sil's species are called the Mentors.
Sil went to the planet Varos to make sure that the governor would lower his price for valuable mineral only found on Varos called Zeiton-7. This would make sure that the Galatron Mining Corporation make a fortune. Sil loved people suffering, dying and most of all money!
Sil uses a language translator to communicate with other beings. Whilest Sil waited for the governor's final offer he watched the excecutions and enjoyed them a lot. These excecutions were held in the punishment dome. Sil wanted the Doctor and his friends badly excecuted as he was impatient. The Doctor and his friends survived even though Sil almost turned Peri and Aretta into a bird and a reptile. Sil however wasn't going to wait for the governor's decision so he asked for an invasion force from Thoros Beta to take over Varos. Sil however failed and the Galatron Mining Corporation would pay an price for the Zeiton-7. The governor made Sil pay so he asked for 7 credits per unit. Sil was furious and he had to pay which lowered the corporations money.Sil returned but the Doctor met Sil on his own planet of Thoros Beta. Sil and the Lord Kiv were meeting a deligate. Sil however was much older than he was on Varos. The Mentors used slaves to clear rocks and some would be experimented on for the Lord Kiv. Sil and the Mentors made a lot of rather odd deals like trading for humans from Thoros Alpha. Sil was later on surprised at the fact that the Doctor was working for him and that he was punishing Peri. Sil howveer didn't know that the Doctor was going to double cross him. Seeing as the Lord Kiv was dying due to his brain growing far too big. When Kiv was being experiemented on and having his brain transplanted if he died Sil and everybody operating on Kiv would die. Sil was under a lot of pressure.
When Lord Kiv's brain was going to be transplanted for the last time Sil was scared incase he died. However Sil said to use Peri as Kiv's new body. When this worked Sil was pleased but was then attacked by King Yarkos but he survived. Peri on the other hand actually did survive as it was a lie.
Sil was suppose to return in another season with Colin Baker in "Mission to the Magnus," along with another old enemy of the Doctor's, the Ice Warriors. But the story was never made.
Sil has appeared in:
Season 22: Vengeance on Varos
Season 23: Trial of a Time Lord - Mindwarp