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 and Abby isn't saying anything either.
There are two, mutually-contradictory origins presented for it. One story is by Gav Thorpe from an old issue of White Dwarf, who says Abaddon found the plans for the Planet Killer in a halo world, then press-ganged a renegade Forge World into building it for him. A more updated story in the 8th Ed Chaos Codex says that during the 10th Black Crusade, Abaddon found an ancient and abandoned shipyard deep in the Eye of Terror that contained the hull of a half-finished ship, which would be the Planet Killer. Regardless, whether Abby had the tech-priests build it from scratch or finished what the ancient owners of the ship started: he eventually completed the Planet Killer by 139.M41 and flew it for a test drive during the Gothic War.
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. Techpriests say that in reality, it is not possible for the Planet Killer to be built, and the reality-bending bullshit of the Warp was main culprit on how it was able to be completed. As to how it continued to function in realspace without breaking apart: it might have something to do with the numerous daemons a Chaos sorcerer named Zaraphiston bound into the vessle.
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.
Beyond the Gathering Storm: Have You Seen My Planet Killer?
Curiously the Planet Killer was 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, was been said about the Planet Killer 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?
The short story The Threshold of Damnation, part of the Tales from Vigilus series on Warhammer Community, sheds a little light on that. While the ship's actual whereabouts and activities aren't revealed, Abaddon notes that while the Planet Killer has its uses, and he's still making good use of it, there's only one true flagship of the Black Legion and that's the Vengeful Spirit. In all likelihood the Planet Killer is probably lurking somewhere out in the Imperium Nihilus, where word of its presence can't easily spread. Following the War of Beasts, it is said to be preparing for an attack on Vigilus' sister world, Sangua Terra.
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, though again the ship's destruction apparently isn't canon, as Abaddon still has the Planet Killer ferreted away somewhere as of his arrival at Vigilus. On the other hand, the events of the campaign as a whole for the most part are non-canon so it all evens out.