It seems... it was the source of his power. Kidnapping and brainwashing people, to fight or die...

[At the least, it convinced friends and family not to attack him head-on, with all those hostages. And... Sans wasn't usually very bestial, but that changed shape of his probably counts. And even then, look what had happened to Julien, or the others; they didn't need to start out all animalish to be that way in the Hunt.]

Though, it called him the 'Huntsman', instead of the Prince? There can't be two Wild Hunts. That would be too many. Right?