Even something as light as a marshmallow is carrying an insane punch at speeds loke that. A small mass at insane velocity approaching the speed of light would tear through a person like a tungsten carbide rod shot from a rail gun. Should it collide with an object capable of absorbing the impact, it would generate the kind of energy typically reserved for fision bombs. At the speed of light or past it, I'm not sure... Extinction level event is a term that comes to mind.