It's Sion, obviously, but at some point in the past, long before present times, Sion was a great warrior and general of Noxus. A favorite of Darkwill, the High General, he was there when Demacian forces, led by either Jarvan the First or Second (since J4 is still the Prince, I think it is safe to Assume Jarvan the Third is still alive). Rather than cower inside the city, Sion led forces against the Demacians, outnumbered though they were, outside the city walls.
Against all odds, Sion reached the king - and did the unthinkable. He slaughtered the royal guards, and fought the king to a standstill. Though he died, Sion achieved his goal, and killed the King as he fell.
A monument was erected celebrating Sion's great will and sacrifice. He was a paragon amongst the Noxian people, one to whom all looked to for the image of what it meant to be a Noxian.
Later, Darkwill sought to resurrect his old friend and comrade, seeking a powerful weapon against Noxus' enemies, and one against his own. The Black Rose saw a way to regrow their roots into Noxian leadership, and offered their services to the High General. What emerged from the ritual was no man - it was an undead monstrosity inhabiting the body of a fallen hero.
There were, ahem,
issues with using the thing in combat. It killed friend and foe alike. It was bad for Darkwill, worse for the Noxian armies, and so, Sion's memory was forgotten, his monument shuttered, and his legacy lost to the centuries. Talk of the hero was forbidden, the mindless body locked away, never to be found or spoken of again.
Until Swain.
Centuries later, the new High General Swain used the notes and tools of the former High General Darkwill to uncover the secrets of Sion. Happening upon the secreted monument and undead thrall, he devised a plan - one that would bring new glory to Noxus. Entering into agreement with the Black Rose once more, Swain sought to give new life to the warrior once know as Sion, but not as Darkwill had. Using the monument as a base, using the bloodline of the King who had killed him, Swain created an undead titan the likes of which Valoran had never seen.
Sion has returned - and the world of the living shall fall before his might.