The Galaxy Gun Edit
For his new superweapon, the Emperor constructed the Galaxy Gun above Byss. Although it was capable of travel through hyperspace, like the Death Stars before it, the Galaxy Gun remained stationed above Byss so that the planet would always be defended.
Even though it was positioned above Byss, the superweapon could still destroy worlds by sending planet-destroying missiles through hyperspace.
Destruction of Byss Edit
Before it was destroyed, the Galaxy Gun fired one of its missiles. However, the missile lost its corse and was pulled toward Byss by the planets gravity. It was this missile that destroyed the planet.