It's the 31st Millenium, you are the first captain of the VIIIth Legion, and after drifting to conflict after conflict with you keeping the Legion together, you get a fateful vox-message.

She's been found, the Primarch of the VIIIth legion, Kassandra Curze.

[Warning: Very hard. You really need to earn every but of trust.]

(You can really do whatever. Be a bloothirsty nightlord and enable her self destruction, be a good boi and fix her, be a horny guy and wife her up. Just know that it's gonna be an uphill battle to form a connection with her.)