From Lexicanum:
Sometimes the rigid constraints of the Eldar Path are intolerable even for an Eldar to bear; such individuals leave their Craftworlds and become known as Outcasts. Outcasts must bear the terrible burden of their heightened Eldar consciousness without the protection of the Eldar path, set free within the universe they are dangerously vulnerable. Only Eldar of especially strong character can survive for long as Outcasts. After years of wandering as adventurers or Rangers, or sailing the seas of space aboard the pirate fleets, most Eldar Outcasts eventually return to the sanctuary of the Eldar path.
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Eldar Corsairs tend to be more active than their Craftworld cousins, who primarily focus on their own defense, whereas Corsairs aggressively seek opportunities to raid. Certain Corsair fleets are closely allied with certain Craftworlds, such as such as the Eldritch Raiders with Iyanden or Xian's Black Raiders with Ulthwé, but others act completely independently. Corsairs may also form alliances with Exodite worlds in exchange for safe haven and military aid. Occasionally, Asuryani Craftworlds and Corsair bands have gone to war against each other. It is speculated that these outbreaks of war are attempts to prevent Corsair fleets from fully falling to the influence of the Drukhari Archons.
Despite this, many Eldar Corsairs are known to frequent Commorragh and a large number of Corsairs are of Drukhari blood themselves. As Corsair ideals and motives differ wildly, their presence may be for pleasure or sport while others may be indifferent to the suffering of the Dark City and seek only profit. Some may even join the Dark Eldar for their Realspace raids.