The interstellar cloud near the Regula planetoid in which the Enterprise and the hijacked USS Reliant fought their final battle, and from which the Genesis Planet was formed.
The Mutara Nebula is the setting for the climax of Star Trek II: The Wrath of Khan in 2285. With the Enterprise badly damaged, Kirk lures Khan Noonien Singh into the nebula, where static discharges disable shields and sensors and leave both ships fighting almost blind.
Spock notes that Khan's tactics show two-dimensional thinking, and the Enterprise wins by attacking from below. The dying Khan then triggers the Genesis Device, and Spock sacrifices his life in the engine room to restore warp power so that the ship can escape.
The detonation reorganises the matter of the nebula into the Genesis Planet, where Spock's coffin comes to rest and where much of The Search for Spock takes place.