Alternate Reality
(split 2233)

The Klingon prison planet was a planet claimed by the Klingons. It served as a prison for enemies and captives of the Klingon Empire.

In 2258, the Romulan mining vessel Narada from the 24th century of an alternate reality, commanded by Nero, attacked the Klingon prison planet. It then destroyed an armada of 47 Klingon warbirds. Those on the planet sent an emergency transmission relating the situation, which was picked up and translated by Nyota Uhura in the long range sensor lab at Starfleet Academy. (Star Trek)

Although not stated on-screen, the Klingon prison planet was meant to be Rura Penthe (which is actually an asteroid). A deleted scene from the film shows that Nero and his crew from the Narada were captured by the Klingons and imprisoned on Rura Penthe for twenty-five years after their ship was crippled by the USS Kelvin in 2233. The attack on the prison planet referenced in the film was Nero and his crew escaping from Rura Penthe and reclaiming their vessel. [1] [2]