I was reading Irelia's background page and it says "A dozen of Swain’s veterans fell, sowing chaos in their ranks as the other captives joined her, before she struck down the general himself—the sight of this rebellious girl hefting his severed arm over her head would be the turning point of the war.". Did Irelia really kill swain and cut his arm off??
Read Swains bio
She cut off his arm. He survived though, just check his bio :p Some weird demon shenanigans were at play!
Well you could have answered your own question if you had just ready Swains bio.
Anyway Swain wasn't the leader of Noxus at that time, he was just one of several commanders of the army, so killing him wouldn't have meant much for Ionia. And overall Swain survived thanks to the demon Beatrice, whom he subdued(somehow, the details for this aren't known).
