The Azure Nebula.
The Azure Nebula was a class 11 nebula found on the border between the United Federation of Planets and the Klingon Empire. It was composed of oxygen and argon, with traces of theta-xenon, fluorine, and sirillium.
It was entered in 2293 by the USS Excelsior during Captain Sulu's ill-fated rescue of James T. Kirk and Leonard McCoy. There he encountered a Klingon battle cruiser commanded by Captain Kang, who attempted to escort Sulu out of the nebula, but was unfortunately disabled when the Excelsior ignited the nebula's sirillium gas.
The Azure Nebula had a superficial similarity to the class 17 nebula encountered in the Delta Quadrant by the USS Voyager that triggered a memory virus to surface within Lieutenant Tuvok. (VOY: "Flashback")