Shrouders were, as their name suggested, the inhabitants of "shrouds", artificially created zones of space so tightly warped that the inhabitants were, effectively, sealed off from the rest of the universe. The shrouds were theoretically impenetrable from the outside -- although they could be deactivated from inside.
The Shrouders began as members of a race of bird-like creatures known as the Amarantin, on the planet Resurgam in the Delta Pavonis system. One group of Amarantin, the Banished, separated from their main society and achieved space flight. Thousands of years later that group inadvertently triggered one of the Inhibitors' devices for detecting intelligent, space-faring life. Unable to escape the Inhibitors, one faction of the Banished created the Shrouds and retreated into them to hide until the Inhibitors -- already declining in ability -- were no longer active, becoming the Shrouders (the other faction chose instead to enter Hades through its moon, Cerberus).
In order to determine if Inhibitors were still active, and thus whether it was safe to leave the Shrouds, one group of the Banished embedded a series of false messages into their Shroud that suggested that the Shrouders were a species that had gathered treasures and hidden them inside the areas of warped space.
Lured by the messages, anyone who penetrated further into the Shroud would have semi-sentient programs and directives embedded by the Banished that would cause the intruders to trigger the Inhibitor device near the Banished home planet. If the Inhibitors responded to the triggering device then the Banished would know that the Inhibitors were still functioning. The program manifested itself as the "Sun Stealer" entity, named after a legendary figure in Amarantin history, which was first imprinted in the mind of Dan Sylveste as he entered Lascaille's Shroud. It was capable of hacking virtually any human computer system and controlling starships, and it later spread to the systems of the lighthugger Nostalgia for Infinity when Sylveste was aboard it. Eventually Ilia Volyova of the Infinity was able to defeat it with the Melding Plague, but not before it was able to activate the Inhibitors.
A second, dissenting group within the Banished planted a separate set of programs and directives to prevent the first group's plan from succeeding, believing that triggering the Inhibitors could lead to the discovery of the Banished hidden in the Shrouds. This agent manifested itself as the Mademoiselle, an entity with similar abilities which was able to penetrate even the Conjoiners' computer security systems, who imprinted itself on the mind of Carine Lefevre when she entered Lascaille's Shroud together with Sylveste.