Fandom: Buffy the Vampire Slayer/Warhammer 40k crossover
Challenge: Alexander Harris, the Lord of War
Summary: YAHF. Xander, Jesse & Willow were all fans of Warhammer 40k growing, and Xander has been building the perfect costume for Halloween. And what is it? A Knight of the Questor Imperialis, of course! And even better, his temporary guest didn't need the Sacristans of the Mechanicus for anything. After Halloween, he remembered it all: growing up as a noble, the Code of Chivalry and Honor and everything needed to stride to battle in a mighty steed again as himself.
Requirements:
1) Xander dresses as an Imperial Knight from 40k
2) He retains everything that character knew from his own life, and what he saw or understood while possessing Xander
3) Everything he keeps is permanent; no getting Giles to wipe his memory or anything
4) Xander takes a larger role in the war against vampires and demons
Optional:
1) He retains physical items from the possession, potentially including the knight suit
2) The knight suit/pilot identified the Mayor's palace as the seat of evil in Sunnydale
3) His costume was from somebody enhanced or otherwise beyond human baseline, and he gets the upgrade himself, plus all the memories of training to control and use it
4) Xander recruits new retainers, using his advanced technology to start winning the war
Forbidden:
1) Xander staying the zeppo, letting Buffy walk all over him
2) Xander dating Cordelia naturally – people with that kind of history would never be able to put it aside enough to function romantically
3) Xander's character and/or equipment being corrupted by Chaos/Evil
4) The Powers That Be as actually Good