Sun Stealer was an artificial entity created by the Shrouders, named and modeled after the Amarantin that first led them to space. Its purpose was to subtlety guide the recently encountered human species into awakening the Inhibitors, to reveal whether they continued to prowl deeper space hunting for intelligent life.
Circa 2418 Sun Stealer was surreptitiously implanted into the mind of Dan Sylveste, during his attempt to penetrate Lascaille's Shroud. In 2460, after the human had been kidnapped onto a powerful vessel, Nostalgia for Infinity, Sun Stealer -- confronted with its hosts vulnerability -- decided to hedge its bets, and infected the lighthugger with a shaved off splinter of its own program. Unfortunately for it, the splinter became trapped in the ship's gunner control systems by a defensive feature of its computer architecture.
In 2491 Sun Stealer's mental influence began bearing fruit, with the archeologist Sylveste heading an expedition to the Delta Pavonis system; the same system as the faux-moon Cerberus which hid an Inhibitor detection device.
In 2543 the splinter program aboard the lighthugger finally made progress influencing reality outside the gunnery systems. Gunnery officer Boris Nagorny was driven insane by its attempts to control him, but the next gunnery officer was infected cleanly -- despite interference from the Mademoiselle, another Shrouder program but with directives antithetical to its own. Eventually, after the main Sun Stealer in Dan was brought back aboard the lighthugger in the Delta Pavonis system, the splinter managed to hitch a ride during a trawl of gunnery officer Ana Khouri, finally gaining access to the ship's main systems.
From there, Sun Stealer and its splinter worked in concert to orchestrate Dan's penetration of the Cerberus object, the former guiding him "on the ground" in a suit of power armour and the latter preventing interruption with Infitniy's armaments turned against its own crew.
At last, however, the pair was brought down. Infinity's captain, previously infected with the Melding Plague, was released from cryo-suspension and regained control of the ship from the splinter -- albeit through exploiting his disease to merge with the ship; simultaneously, Sun Stealer was destroyed when Dan killed himself -- via an antimatter explosion -- to prevent the activation of the Inhibitors.