Sleepers

Sleepers were a faction in Heat Signature during it's development. They were eventually removed from the game, but persist in the lore of the universe, being referenced by Breaker Gemini during conversation.

Background
The Sleepers appeared in the Drift before the events of Heat Signature, speaking a language and using technology completely unknown to it's inhabitants. They attempted to invade the Drift using their massive force and looked set to succeed until interference by Sader Fiasco.

According to Breaker Gemini's recounting of the story, the Sleepers killed someone close to Fiasco during their invasion, which caused her to step in in the first place.

Sader Fiasco single-handedly assassinated the Sleeper commander within an hour of her assault, and over the following years stole 'half their fleet', and leaked their ciphers.

The Sleepers were further weakened by the arrival of Sovereign, and eventually finished off by an electrical storm. Both events that are believed to be the work of Sader Fiasco as well, though it is unclear exactly how.