The Craftworld Eldar are some of the remains of the Eldar race after the birth of Slaanesh. They travel the void on large ships housing the entirety of their respective worlds within them. Most consider themselves as the last protectors of the universe, patrolling and making sure that Chaos and the Necrons cannot attain the power they desire.
The Craftworld Eldar’s relations with the Imperium is strained at best. Most of the time the Eldar show up at a moments notice, fight to attain whatever goal they think is necessary, then leave. In general, they fight with the Imperium against it’s threats, but it is just as likely for them to attack the Imperium for expanding it’s reach farther than the Eldar believe it should.
Little is known about the various Craftworlds, as the Exodites left before they were constructed, and most of those on the Outcast’s Journey refuse to let the ’Mon’keigh’ learn about their secrets. Those on the Outcast’s Journey do aid the Imperium, or whichever Rogue Trader that happened to pick them up, with whatever they wish, but most still keep their own agenda to help protect the universe from the ancient and vile evils that lurk around every corner.