The Arcane Mystery of the Other was a time dreadnought in a parallel universe where the Sixth Doctor led the Time Lords in the War. It was commanded by the that universe's equivelent of the Master. Originally, when he defected to the enemy, it was assumed lost. In reality, it was dissected in an enemy shipyard. (PROSE: The Quantum Archangel)
Behind the scenes
The ship was named after the legendary time lord, The Other, with the added description referring to the namesakes' mysteriousness.