This is based off the old lore for Sion, where Katarina retrieves him after being captured by Demacian forces. There's more to it (can look it up on the wiki if interested) but here is the barebones for this story:

"The Demacians thought that the beheading of Sion would be the end of his bloodthirsty ways. Death, however, was merely the beginning of Sion's rise to power. The famed Noxian assassin,  Katarina, recovered Sion's remains and Noxian necromancers reanimated the behemoth to serve Noxus once again." 

I'd like to think I've put a fair bit of effort into this one, any criticism would be greatly appreciated. This story spawned from my interest of vore and the thought of "How did Katarina manage to carry the massive oaf that is Sion all the way back to the Noxian base?" Enjoy.