Nightmare is a map available in Halo: Lone Wolf. It is clearly based on the campaign version of Powerhouse, with the Covenant Stealth Pylon having been dropped.
An asymmetrical map that takes place in the Covenant occupied UNSC powerhouse. The Covenant Stealth Pylon sits active, suggesting that SPARTAN-B312 and Jun-A266 haven't liberated the base yet. The bridge hasn't collapsed yet (however, players who attempt to leave will encounter a soft kill zone), and a Phantom looms beside the Stealth Pylon. The map features basically the same weapon and player spawns of Powerhouse.