"According to the mystic Cornelius Agrippa, he is the prince of 'Gehenna', one of the seven levels of hell. His name means 'the sun of night'. He is commonly seen as one and the same with Abaddon and Satan."
—Shin Megami Tensei III: Nocturne Compendium
Alciel can be obtained by evolving Abaddon. However, he must be defeated as a Boss in the Black Temple section of the Amala Temple first before obtaining him through transformation.
"The "Black Sun" who lives in the underworld in Babylonian lore. His original name is Aciel, which the Jews borrowed from a Babylonian god, and called him the "King of Gehenna." But in some Asian faiths, all things are reborn only in darkness, so Alciel is not all bad."