Meph said:
Have the characters slowly find out details that makes them thÃnk that they're descendants/incarnations of the primarch but instead of it being pure and divine as they think, let it be the machinations of chaos. It seems like a perfect thing that Tzcheench would want to pull off. Gradual mutations, visions of perfection, eveolving into the pinnacle of humanity, only to have it turn sour and have the characters trying to claw back their way to humanity or making the hard choice of heroic sacrifice. It would make an amazing apotheosis to a long campaign... Be tempted by chaos and the promise of daemon-princehood or defy the ruinous power at great personal cost but saving the galaxy from an even greater horror.
I too like this idea, but I think I'd like to do a variant of it. The player characters are indeed the descendants of primarchs, and the primarchs' souls have fused with the PCs' souls. This could have a variety of effects, such as strengthening the powers of psykers or navigators, sharpening the will, making someone more/less resistant to corruption and insanity, or causing someone to become a nascent psyker. However, it wouldn't cause someone's entire body to grow new organs and gain a couple feet in height. The ruinous powers can see that each primarch soul has now fused into one of the PC's souls, and the only way to get back those prizes is to kill the PCs and take the new amalgamations. However, they see this as an opportunity, and start juicing up the PCs so that mutate in a fashion appropriate to a primarch. Cue temptation, conflict, and manipulation by the ruinous ones as per Meph's post.