In the Nightside Eclipse is the first studio album and fourth official release by Norwegianblack metal band Emperor, released in 1994 through Candlelight Records. It was the last album to feature drummer Faust and the only one to feature one-time bassist Tchort. Considered a landmark in the black metal scene, the album has been ranked by critics as one of the most influential albums of the genre. It also contains one of Emperor's best known tracks, "I Am the Black Wizards".
In his review, Steve Huey of AllMusic gave the album five stars out of five, writing, "In the Nightside Eclipse resoundingly demonstrated that there was real musical substance and ambition in the world of black metal. [...] [it] somehow managed to capture the essence of the genre while completely rewriting its rule book", commenting that it was also "the first [album] to fuse black metal with progressive and symphonic elements, setting the stage for a bevy of future experimentation in the genre, [...] As such, it certainly possesses the farthest-reaching legacy of anything from Norway's bloody first wave, and ranks as one of the most important heavy metal albums of the '90s." In 2005, the album was ranked number 292 in Rock Hard magazine's book of The 500 Greatest Rock & Metal Albums of All Time.
In 1999, the album was remastered and reissued, with two cover songs as bonus tracks: "A Fine Day to Die" by Bathory, and "Gypsy" by Mercyful Fate. For the reissue, the opening tracks "Intro" and "Into the Infinity of Thoughts" were combined, whilst the album was packaged in a paper slipcase covering the traditional jewel case, with both featuring the same artwork. A second reissue followed in 2004, which included videos of live performances from 1997.
In 2009, IGN included In the Nightside Eclipse in their "10 Great Black Metal Albums" list; according to IGN, "Emperor inspired the wave of overtly-technical black-metal bands that would rule the underground in the early 2000s. Dimmu Borgir and Cradle of Filth owe a huge debt to this album."
In July 2014, In the Nightside Eclipse was listed at number eleven in Guitar World magazine's list of "Superunknown: 50 Iconic Albums That Defined 1994".