Oof. Sorry but you are absolutely not up to date with grimdank memeology.
The eldar woman is Yvraine from craftworld Biel-Tan, she is a prophet for the new eldar goddess Ynnead and goes around the universe trying to find new followers. She's in a sect, in short.
She stumbles upon Belisarius Cawl and helps him revive Roblox Gullible (nobody says Guilliman, you fool).
From that simple thing, people have gotten quite creative in terms of dreaming a relationship between Yvraine and Rubik's Guacamole. Yes many rule34 depravations have naturally followed.
To sum up, in a world where every major hero from 30k has more or less disappeared, the romance of Yvraine and Robust Guitarist is the only chance to repopulate the galaxy with hope and beautiful babies