The Living Force
The c^2 part of that equation is pretty important. Direct conversion of 90% of the mass of an entire planet to energy wouldn't just destroy the planet. It would destroy the solar system, including the Sun, and probably every star in the solar neighborhood. It's far more energy than is released in a supernova.
This discussion of Kantek, energy, and how it was destroyed brought to mind the destruction and dustification of the Twin Towers on 9/11. There was a lot of rubble left over, but a lot of the material just went puff. Maybe there are similarities between how the Twin Towers were destroyed and how Kantek was destroyed.