Answer by taking a quote from the following article:

Therion (formerly Blitzkrieg, Megatherion) is a Swedish symphonic metal band founded by Christofer Johnsson in 1988. Its name was inspired by the Celtic Frost album To Mega Therion. "To Mega Therion" is Greek for "The Great Beast" and was a title used by occultist Aleister Crowley. Originally a death metal band, Therion adjusted its musical style by adding orchestral elements, including choirs, classical musicians, and even a full orchestra at their concert performances.

After the Secret of the Runes tour, the band had amassed 55 unreleased songs which included the ones Johnsson had shelved as well as contributions from the Niemann brothers. Johnsson remarked that "we went through the songs and realized that we had enough good material for three albums. Recording one after the other, promoting, and touring each one separately would have taken several years and would have been a bad idea; and being so productive, we would have had another 55 songs waiting when we finally came to the end of it." In 2004, the band released 21 tracks on two albums: Lemuria and Sirius B. The albums were released simultaneously and were available as separate albums or a twin-pack. 171 musicians participated, including the City of Prague Philharmonic Orchestra and a 32-member choir. Contributing to lead vocals were Mats Leven and a returning Piotr Wawrzeniuk. In July 2005, the band released Atlantis Lucid Dreaming, which is a compilation of the 1997 tracks that were not part of The Golden Embrace soundtrack (not including its Iron Maiden cover of "Children of the Damned"), and seven tracks from its 1999 album Crowning of Atlantis. It included a bonus live version of "Black Sun". The band toured Lemuria / Sirius B over two years and 106 shows, with its final performance at the ProgPower Festival in Cheltenham, UK on March 21, 2006. The last gig marked Johnsson's final vocal stage performance. Therion released their first DVD set, Celebrators of Becoming, on May 2006. The set contains four DVDs which include concert footage from its 2004 visit to Mexico City, documentaries of its 2004-2006 World Tour, Johnsson's art movie The Golden Embrace, the band's music video singles, bootlegs and commentaries, and two audio CDs from its Mexico City concert.

When were they released?
In 2004, the band released 21 tracks on two albums: Lemuria and Sirius B.