From Lexicanum

Planet Killer was the flagship of Abaddon the Despoiler during the Gothic War.

The origins of the massive stellar vessel known simply as the Planet Killer are a mystery to Naval authorities. It bears no resemblance to any Imperial ship design and is assumed to have been constructed within the Eye of Terror by Abaddon’s forces, just prior to the Gothic War – in fact several of the Adeptus Mechanicus doubt whether its construction would have been physically possible outside of warp space. Only one Planet Killer is known to have been built. Built around a central energy cannon of immeasurable magnitude, the Planet Killer is also studded with numerous long range lances, weapons batteries and torpedo launchers.

Well armoured and protected by a plethora of shield generators, the Planet Killer’s only weakness seemed to be its ponderous speed. It was this lack of pace which was to lead to its eventual downfall, as it was destroyed by salvo after salvo of torpedoes, fired at extreme range by the Lunar class cruisers of battle group Omega, shortly after the destruction of Kharlos II.