Answer by taking a quote from the following article:

Celtic Frost () were a Swiss heavy metal band from Zurich. They are known for their strong influence on the development of extreme metal. Formed in 1982 as Hellhammer, the band became Celtic Frost in 1984 and was active until 1993. It re-formed in 2001 and disbanded following frontman Tom Gabriel Fischer's departure in 2008.

One of their more influential recordings was 1985's To Mega Therion which did not feature Ain on bass, but stand-in Dominic Steiner. The cover artwork is a painting by H.R. Giger entitled Satan I. The album was a major influence on the then-developing death metal and black metal genres. Ain did return after the album was recorded however. In 1987 followed Into the Pandemonium. The album is more varied than many of Celtic Frost's past LPs, with unlikely covers (Wall of Voodoo's "Mexican Radio"), emotionally charged love songs, the album's recurring industrial-influenced rhythmic songs of demons and destruction, traditional Frost styled songs about dreams and fear, and a dark, classical piece with female vocals.  The album is vastly different from the band's previous work and cemented its late 80s avant-garde metal term; it is also a departure from the extreme style found on the band's previous albums, Morbid Tales and To Mega Therion that Celtic Frost had become known for. However, it does have the recurring symphonic elements found on previous albums. The album has a more classic heavy metal style within the songs with elements of industrial, classical and gothic rock, and even has an industrial/electronic body music (EBM)-inspired rhythm in "One in Their Pride". It does have a few black metal elements remaining in Warrior's vocals, though, and some thrash-influenced guitar riffs.  These albums were some of the pivotal LPs for underground metal and introduced a new and more varied sound. Celtic Frost, along with Venom and Bathory were pioneers in the still underground black metal scene, although Celtic Frost were much more experimental with the addition of classical instruments, operatic female vocals and sampling. Celtic Frost was often labeled by critics as avant-garde metal.

Are there any other interesting aspects about this article?
Frost styled songs about dreams and fear, and a dark, classical piece with female vocals.