Warthur
Although Celtic Frost's earliest albums were incredible blueprints for the more "evil" brands of extreme metal (death metal and black metal) to follow, their later efforts weren't so influential. Into the Pandemonium, whilst intriguingly experimental, didn't really inspire anyone to follow suit, whilst Cold Lake and Vanity/Nemesis found the band following the glam metal and thrash metal pack respectively.
Monotheist, their reunion album, is another question entirely. After decades of other bands building on the mighty foundations erected on such recordings as Morbid Tales, Emperor's Return, and To Mega Therion, Celtic Frost's return finds them synthesising more or less every extreme metal genre imaginable into a horrifying chimera. The centre of gravity of the album's sound seems to be rooted in doom metal - but of a distinctly modern bent with little Black Sabbath influence - but so much of death metal, black metal, gothic metal and even progressive metal (particularly on the closing triptych) seems into the music that it's incredibly hard to pin the album down.
What's even more impressive is how well this delirious mishmash of genres works. It's clear that Celtic Frost haven't settled for any mere nostalgia-fest from the reunion; instead, they've produced an album which sounds incredibly ahead of this time, showing the young imitators once and for all who the real kings of extreme metal are. We may never know the exact reasons behind the final Celtic Frost split - Tom Warrior isn't letting any hints slip beyond his disappointment at Michael Ain for suggesting that the band might continue without Warrior, and his barely-concealed rage at drummer Franco Sesa who Warrior seems to primarily blame for the split - but we can at least be grateful they went out with a bang. In fact, I'd say this is the greatest album they ever made.