Editing Celtic Frost (section)

===Reunion and ''Monotheist'' (2001–2008)===
[[File:Tomwarrior celticfrost.jpg|thumb|right|200px |Tom Warrior performing in 2006.]]  
In late 2001, Fischer and Ain began writing music together, with Unala on guitar and, soon after in 2002, Swiss drummer Franco Sesa. Their goal was to develop and record a new dark and heavy album. The completion of the project took longer than anticipated, partly due to the DIY nature of the project and its financing, but it ultimately resulted in what Fischer and Ain described as "perhaps the darkest album Celtic Frost has ever recorded."

The final Celtic Frost album was financed by the band through its own label, Prowling Death Records. This label had originally been self-founded to release the Hellhammer demos and manage Hellhammer's career in 1983 and 1984. The album was produced by Celtic Frost with [[Peter Tägtgren]] and mixed by Fischer and Ain. Celtic Frost and Prowling Death Records eventually entered into a worldwide licensing deal with Century Media Records. The album, titled [[Monotheist (album)|''Monotheist'']], was released in May 2006. It was well-received by both critics and fans of the band. While experimental and heavy in nature, it is viewed as an evolution from the extreme metal styles first developed on ''Morbid Tales'' and ''To Mega Therion'', as well as the avant-garde style of ''Into the Pandemonium''.

On 29 May 2006, Celtic Frost embarked on the most extensive tour of the band's career, the ''Monotheist Tour''. They initially headlined festivals (e.g., [[Wacken Open Air]]) across Europe and the United States/Canada in 2006, followed by the group's first-ever shows in Japan in early 2007. The European leg of the tour took place afterward, followed by a return to the United States to complete the tour with [[Type O Negative]]. Additional festival appearances and concerts continued in mid-2007.

On stage, Celtic Frost performed with an additional touring guitarist to handle [[rhythm guitar]] parts. This position was initially filled by [[Anders Odden]], and later by V Santura.