Meh. Of course anything relativistic would be orders of magnitude more destructive, I'm not THAT clueless But if the goal is terrorist attack about killing a whole bunch of people, between "razing anything on the surface of the world" and "fracturing the planet", the result for people is not a whole lot different, even though the second is millions of millions times more powerful. Regardless, the destructive energy of relativistic impact is precisely why it's considered impossible to reach (because, well, you'd need infinite energy to reach light speed) so the question is rather moot because you would still need to produce that energy to begin with (which could be used to bomb the planet too if it could be produced). That's already something we can theorically do, and you "just" need to mount a big thruster with enough fuel on an asteroid to do it (okay, the amount of fuel is not going to allow 1/3c, but it'll be fast enough to do a LOT of damage). Or a big railgun and a lot of calculation, and you fire a 100 tonnes slug from Pluto in a direction where it'll use slingshot effect to accelerate and end up hitting Earth. Again, if you can destroy one third of the Earth, it's functionally about the same as cracking the planet for killing people purpose. Well, there is a reason I started my sentence with "if" Reaching speed of light requires infinite energy according to physics, so going past that kind of violate the laws, unless you can find a way to produce "more than infinite".