War Hammer was a Pellaeon-class Star Destroyer in the service of the Imperial Navy.
In 137 ABY War Hammer was commanded by Moffs Rulf Yage and Fehlaaur'aitel'loro and served as the flagship of the First Sith Imperial Strikeforce. The vessel, along with the task force, was ordered with capturing Emperor Roan Fel and his daughter Marasiah Fel during their meeting with New Jedi Order representatives on the planet Agamar.
During the ensuing battle, the War Hammer destroyed the Dauntless, a Fel loyalist Star Destroyer under the command of Admiral Tohri Challon, but failed to stop the Emperor's shuttle from escaping to hyperspace.
The War Hammer, with Vul Isen and Darth Azard aboard, later poisoned the seas of Da Soocha and attacked Napdu, where the Hutt Azzim Anjiliac Atirue had been hiding Mon Calamari refugees from the similar poisoning of their homeworld.
The War Hammer later led a strike on the Hidden Temple, a secret Jedi refuge located on the planet Taivas. Initially Sith-Imperial forces attempted an orbital bombardment on Taivas, but magnetic interference from Taivas disrupted their targeting computers to the point that there were friendly-fire casualties while no damage was done to the Hidden Temple. The Sith-Imperials began a ground assault under the command of Moff Geist, and engaged both Jedi and Imperial Knights protecting the temple. Meanwhile Galactic Alliance and the Empire-in-exile fleets arrived and and trapped the Sith-Imperials between themselves and Taivas.
Krayt chose than moment to deploy his personal force of Sith troopers. These arriving troopers attacked both allies and enemies, killing Geist and a large number of others on both sides in to the process. When ordered to destroy a ship full of Jedi younglings by Darth Rauder, Captain Yage instead killed Rauder and defected to Fel's forces.
Rather than kill his own daughter as ordered, Yage killed his own Sith overseer, and then announced that he and the crew of the War Hammer were defecting to Fel's forces. A number of other Sith-Imperial forces also defected. The Alliance, Jedi, and Empire-in-exile forces fled to Bastion with their new allies.