The Shadow
Planetkiller is not really a ship per se. It is composed of a cloud of particles (presumably
nanotechnology) and a huge superstructure capable of launching huge missiles.
When attacking a planet, the superstructure and cloud surrounds it, devouring the atmosphere and everything in it, while thousands and thousands of missiles are lauched towards the surface. These missiles drill down into the surface and explode, gradually tearing the planet apart.
The cloud can also be effective against ships, where the cloud particles attach themselves and disable vital ship systems.
To date, there have only been 4 in existence. It is presumed that these "Death Clouds" were created at a Shadow Base called Xha'Dam.
The First Death Cloud was seen in 2261, and attacked the Army of Light at Coriana 6 and eventually left Beyond The Rim at the end of the Dawn of the Third Age of Mankind.
After the Shadows left, Beyond the Rim, the Drakh found Xha'Dam and began construction of three more Death Clouds.
The First was used to attack Earth and the Earth Alliance in "A Call to Arms", it was destroyed by Interstellar Alliance President John Sheridan (Aboard the Victory Class Destroyer Excalibur) and Earth Force Captain Anderson (Aboard the Victory Class Destroyer Victory.) The Victory was lost in the battle.
The Second and Third were destroyed by Vir Cotto and a group of Techno-mages in 2268. ("Legions of Fire: Armies of Light and Dark")
The Shadow Planetkiller was first seen in the episode "The Long Night", where it suffocated and ripped apart an entire planet.
The First Shadow Planetkiller left to go Beyond the Rim in "Into the Fire"
The Drakh controlled the Second Planetkiller in "A Call to Arms"
The Drakh planetkillers were also featured in The Centauri Prime Trillogy.