Styx

A small ice and snow-covered moon orbiting Charon, a vast gas giant, is home to the Decepticon penal colony called Styx. Of all such installations it is the harshest and most brutal. Prisoners sent to Styx have a short life-expectancy and may be better off for that. The penal colony is said to be impregnable.

Gutcruncher was stationed there for a time, before he was on Banzaitron's team.

IDW Generation 1 continuity
At some point during the war, all Decepticons imprisoned at Styx were re-engineered into living bombs and forced to join the K-Class. Among these individuals was Fulcrum, who was awaiting execution by Traitor's wheel for running from a battle.

Hot Rod impregnated the impregnable prison (get your mind out of the gutter) with a virus that let him sneak in and free his former comrade, Dealer. Although Dealer seemed appreciative, he failed to give Hot Rod a "".

The Decepticon battalion commander Gorelock was imprisoned there for giving aid to Overlord.

Ask Vector Prime
In Primax 490.0 Gamma, the prison moon Styx held the few remaining Decepticons. Jhiaxus was dispatched to retrieve the Decepticons so that a unified Cybertronian species could fight Unicron.

Animated cartoon
Styx was an organic moon in the Decepticon Empire.