In the Nightside Eclipse is the debut studio album by Norwegian black metal band Emperor, released in 1994 through Candlelight Records. It was their only album with drummer Faust and 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 contains some of Emperor's best known tracks, "I Am the Black Wizards" and "Inno a Satana".

The album was co-produced by Pytten, who produced Mayhem's De Mysteriis Dom Sathanas and Burzum's debut album, as well as albums by Immortal and Enslaved. Faust partially credits Pytten with the sound of In the Nightside Eclipse.

Most of the music was written and rehearsed before the band entered the studio. However, much of the symphonic keyboard sections were composed in the studio at the time of recording as the band did not then have a permanent keyboard player.

Ihsahn was 17 years old at the time of the album's recording, and because he was unable to join the rest of the band members in festivities at the local "rock pubs" in Bergen after the album's recording sessions, he spent large amounts of time in the studio working with audio engineer Pytten. Ihsahn had a preexisting interest in audio engineering and took instruction on recording technology from Pytten.

Although the album was recorded in July 1993, it was not mixed until the following year due to Faust and Samoth's arrests and sentences in jail. In the end, only Ihsahn and Samoth were present for the mixing of the album, though he passed along his input to them in a letter. Ihsahn was ill at the time of recording, and Tchort recalls him spitting blood while recording vocals for the album. Some of the early vocal takes were replaced with ones recorded after he had recovered, as were some of the keyboard parts.

Pytten would later be quoted as follows regarding his feelings on the finished product: "With such long times working on the production, so many hours in the studio, so many replays of the songs, so many tries to get the music right, I can go on... I have to admit my first feeling was relief! But the way I see the album after the fatigue left me is that I have never thought, 'Oh if I only had done so and so instead...' I think, whatever words are put on the production, this is a captured sound that has a lasting quality. I am quite proud of what we all achieved."

The album cover was drawn by Kristian Wåhlin, also known as "Necrolord", depicting a host of orcs en route to Minas Morgul. The part below the band logo is based upon a section of a larger engraving called Death on the Pale Horse by Gustave Doré. That section itself was also used as the album cover for the Emperor (EP).^{[citation needed]}

The interior artwork features a photograph of the Trascau Fortress in Transylvania.^{[citation needed]}