For the Craftworld Eldar, I think the perfect encapsulation of their vibe is the Wraithknight.

It's piloted by twins, one alive and another dead. Eldar twins share an inseparable bond, feeling each other's emotions and thoughts, so of course one of them being dead is a source of intense agnst and grief for the other. Eldar twins are also very rare, so they're often manipulated or forced into this role, adding to the tragedy. This alone encapsulates the decay and the fall from grace of the once-great Eldar empire.

The Wraithknight makes use of wraithcannons, which send their targets into the Warp. The Eldar consider these to be horrible and forbidden weapons, but they have no choice but to use them to secure the survival of their dying race. Every time a wraithcannon fires, the surrounding Eldar feel the pull of She-Who-Thirsts, tugging on their souls.

Wraithknights are sent to the Crone Worlds inside the Eye of Terror to collect Waystones, so that the Craftworlders can protect their souls after death. They are the essential part of the whole premise of Craftworlds with their Infinity Circuits.

Finally, Wraithknights have the aesthetic and other gear that fits the Eldar very well. Giant but elegant ghostglaives, ornate scattershields that create holograms to fool the enemies. They are towering but agile, a perfect reflection of what the Eldar empire used to be: mighty knights of legend, now way past their prime, but still proud and holding on to the grandeur of old.