Here is the fluff of my Craftworld. I'm still working on it so tell me what you think.
The Treventhius Eldar takes their name from a contraction of ancient scroll called the ‘Treventhi Arcturus’, but was not always called this. In the scroll there is a prophecy that states that a group of Eldar will find the path of salvation for the remnants of the Eldar race after the great catastrophic event that is to happen at the height of the Eldar reign. This apparent path can only be found by risking all against ‘She that thirsts’, so that these Eldar must be of tremendous psychic power to guide the rest into salvation. The scroll states that these Eldar will not be the same Eldar that re-unite the remaining Eldar, this is left to another.
When the ancients joined their ships and created the more widely known Ulthwé Craft world, the Treventhius Craft world was created at the same time as a sister craft world (why build one, when you can build two). These ancients knew of the prophecies of the ‘Treventhi Arcturus’, and guided the two craft worlds to follow the path of the seer above all else. As the Far seers of both craft worlds increased in power and wisdom, it was foreseen that the Treventhius Eldar would be those Eldar spoken about in the scrolls. Their craft world’s name was changed to Treventhius in respect of the scroll, losing its original name to the warp. The scroll is rumoured to be stored at the Black Library, but can be seen in a special anti-room on the Treventhius Craft world where only the most powerful Far seers from Treventhius and Ulthwé can enter.
Over the course of time, they have had more dealings Ulthwé then any other craft world because of their same ancestors, and rival in numbers the Eldar who follow the Path of the Seer and the Path of the Warlock. It is said the some of the Treventhius Seers are more powerful then Eldrad Ulthran himself. Since the craft world’s creation, many of the Treventhius Eldar has been mistaken for the Ulthwé due to their similar markings and numbers of psychics seen in battle. This may lead to why the Ulthwé is wider known throughout the galaxy.
The Treventhius Craft world follows a house system, with the leading house being house Treventhius. Every Far seer on the craft world belongs to house of Treventhius, as well as the majority of the warlocks. This is due to when an Eldar outside this house shows to be of significant psychic strength, they are made part of the Treventhius family. It is a high honour to be made part of the Treventhius house, and is extremely rare for someone to decline. Those who do decline are not discriminated in any way and can grow in strength to become a Far seer, but can never become a member of the ruling body of the Craft world, and are rarely seen in battle.
Because of the numbers of Eldar who do walk the path of the seer and path of the Ulthwé, there are always a large number of warlocks seen in battle. The Treventhius Seer council that leads their brethren into combat always has a minimum of two warlocks per far seer in the council. A warlock also accompanies every Guardian squad when possible, be it Defender, Storm, Support Weapon or Jet-bike. The later is less likely to be seen, as the Treventhius seers are far to old and proud to ride jet bikes which discourages many of the aspiring Eldar.