Valhalla is the background music that plays on the G.F.S. Valhalla in Metroid Prime 3: Corruption. It is available for purchase in the Soundtrack Gallery as well. Internally, it is referred to as Dark Sad Kowa. It should be noted that "kowa" means "fear" in Japanese.
The Valhalla has two different musical themes. The first plays until Samus Aran initially enters the Aurora Access room, which requires the Nova Beam. Upon doing so, the second theme, Abandoned Ship, replaces the first for the remainder of the game.
Valhalla is an ominous orchestral track, primarily composed of string instruments an other atmospheric effects, while the other is an extremely eerie track that primarily consists of ambient noises. Both themes add to the already haunting atmosphere of the Valhalla itself. Interestingly, the first one's composition slightly resembles that of Environmental Sound (Disquieting) from Metroid Fusion.
The theme can be heard here.