The Execution Eternal is an ancient Oberon-class Battleship lost to a crew mutiny in the early ninth century of the 41st Millennium while on a mission to reinforce Imperial armies fighting on the Achilus Crusade's Greyhell Front.
The ship's officers were murdered, but not before they could sabotage the Warp-Drives, preventing the mutineers from fleeing with the ship. T'au forces learned of the ship's plight and sent a Hunter Cadre to capture it, but the Imperial Navy sent its own mission to recover it and the resulting battle raged through the countless decks of the starship.
It is still going on, with the T'au holding the cavernous engine chambers and the Imperial Navy troops holding the bridge and crew decks.
Neither side can gain the upper hand. The city-sized ship is a void-bound battlefield, the two forces fighting a microcosm of a war through its body-strewn corridors.
The capture of the Execution Eternal is now a matter of pride, the symbolic value of its capture far greater than the military value of the ship itself. The Imperial Navy considers this its own fight and refuses to allow Astra Militarum troops to join their security battalion troops on board.
For more than a standard year, Human and T'au have fought, every crew berth and bulkhead door won and lost in blood, and unless the stalemate is dramatically broken there is no end in sight for this ship-bound miniature war.
- Deathwatch: Rites of Battle (RPG), pp. 250-251