Given the description from the League of Legends Cotton Face Mask 3D USA, my personal theory is that the fox and the demon represent two halves of Yone in the afterlife, one leading him to peace and the other consuming him with the failures of his past. Yone chose to be consumed by his own demon. This also is why the demon only wields one sword when Yone has two, because the demon is only half of himself.
League of Legends Cotton Face Mask 3D USA
Best League of Legends Cotton Face Mask 3D USA
If you watch the League of Legends Cotton Face Mask 3D USA, he seems to awaken some energy or such when the blade is coming down on him. His eyes glow and blue horns grow from his temples. After stealing the blade, before the strike, he emits energy and the blade changes form. It becomes gold with a blue edge and the horns at his temples are longer still, then he strikes the darkin(?) down and still has the horns before being struck. It seems he conquered the blade for a moment.