Space Marine chapters are all well and good, but as an Eldar player I'm more interested in people's craftworld backgrounds; how they relate to established Eldar lore, and how they're unique.
My craftworld, Eli Sheol, drifts vulnerably through the Deicide Sector (also made up - where all my Dark Heresy stuff happens) in Segmentum Ultima, a microcosm of the Eldar's war against their kin. They suffered a catastrophic webway breach to Dark Eldar interlopers, and so the graceful gardens, domes and spires of Eli Sheol regularly roll back and forth in a protracted tug of war of urban conquest.
The survival of the craftworld as a viable host has played an important enough role in several prophetic rune casting to warrant direct military aid from the craftworlds which still have unbroken connections to this one - Ulthwe among them, neatly justifying Eldrad's occasional cameo.
Paintscheme is classic skull white and shadow grey. What about your craftworlds? How does your splendor shine so that it terrifies and awes the mon-keigh?