Sil was basically a slug on a box that did a weird tongue laugh thing that everyone though was super creepy. He was kind of a jerk and kind of  a pervert. He also turned Peri into a bird lady.

 

He was the representative of the Galatron Mining Corporation and hung out on Varos desperately trying to get at the mineral Zeitron-7, much to the dismay of local Varisuans. He also showed up at the Doctor’s trial by the Valeyard.

He was seen in Vengence on Varos, and Trial of a Time Lord.