Nice. A white Overrun, reminiscent of Titanic Ultimatum or Flying Crane Technique. The option to get it for very cheap makes it feel mythic. The vigilance is very sensible but makes me think this should be a sorcery.

It could be, but there's a slim chance you might need to buff your creature's and give them first strike to block, so I have it at instant.

And sometimes I wish I could Overrun on my opponent's turn as a combat trick, but that doesn't mean instant speed is a better design. Vigilance on an instant is a bad idea.