Planet Killer
The Planet Killer is a Chaos battleship that was used as Abaddon's flagship during the Gothic War. It is infamous for its main armament, the Armageddon Gun, whose five barrels can lay waste to entire worlds.
In-Universe History
No one knows where Abaddon managed to pick the Planet Killer up (there are two, mutually-contradictory origins presented for it), but the ship gained infamy among the Imperials for being the Despoiler's flagship during the entirety of the 12th Black Crusade. Its armaments ended the cardinal world of Savaven, while the debris field that once was the three moons of Stranivar bear eternal witness to its wrath.
During the Eye of Terror Campaign
Prior to the lore changes wrought by the Gathering Storm, the ship participated in the 13th Black Crusade, where it destroyed the worlds of Macharia and Urthwart. Unlike during the Gothic War, Abaddon didn't use it as his flagship; instead it was loaned to another Chaos Lord named Malefica Arkham, who used it to devastate the planets of the Arimaspia system.
While Abaddon was kept busy at Cadia, Arkham decided to use the Planet Killer to carve out his own small empire, and conquered the worlds of Gonnacrash and Quinrox Sound. At Kharlos II however four Lunar-class cruisers got the drop on him, and without any escorts the Planet Killer met its end.
Of course, that's been changed since then.
Have You Seen My Planet Killer?
Curiously the Planet Killer's nowhere to be seen in the lead-up to the destruction of Cadia, its place taken up instead (fittingly enough) by the Vengeful Spirit. Nothing, not even a rumor of its sighting in-universe, has been said about in the hundred plus years since the Great Rift formed, the Indomitus Crusade was launched, the Plague Wars wracked Ultramar, and the War of Beasts hit Vigilus. So where the heck is it?
In Battlefield Gothic
The Planet Killer is a souped up Chaos battleship, with the addition of the Armageddon Gun as a special weapon that hits everything in front of it, friend or foe alike.
Hilariously, despite being described in-universe as "slow" for a battleship, rules-wise it's still much faster than some Imperial battleships like the Apocalypse-class.
In Battlefleet Gothic: Armada
The Planet Killer shows up as part of the final "boss fight" in the game, in the historical battle of Schindelgheist. You can get to destroy it in-game, though historically Abaddon and the ship manage to beat a hasty withdrawal back to the Eye of Terror.
It shows up again in the sequel, as part of a trap that one of Abaddon's Chosen and the Iron Warriors spring on Admiral Spire. Fortunately for the latter, the Macragge's Honour shows up as reinforcements, and thanks to Bobby G's help, Spire is able to destroy the Planet Killer once and for all.