I see more and more bands come crawling out of the woodwork who vanished many years ago. Two that immediately come to mind are Hell (UK) and Heavy Load (who went forty years between albums!!). There have been a bunch in the last ten years or so, though, and I was looking for some more examples. They didn't have to have great comeback albums, but I've noticed many do. No real rules, but I'm not looking for King Diamond or other bands that have been supposedly working on an album for a long time.

Cirith Ungol had 20 years between Paradise Lost and Forever Black, and their reunion stuff had been just as good as their original run

Satan, the 80s band released a pretty cool comeback album at the height of the traditional heavy metal revival.

Morbid Saint is another one, they put up a classic album in the 90s and disappeared till last year when they released a new album.

Afterbirth is other than I can think of, they released a few brutal death metal demos in the 90s and then waited till last year to release a pretty experimental and psychedelic bdm album.

Adding Job for a Cowboy to the list, they skipped a decade before releasing the modern masterpiece that is Moon Healer.

I'll always give the band a plug, Solstice were 20 years between LPs, and 15 between any official release.

Pagan Altar have actually done this twice. Firstly between the debut record/album and then rereleasing it again 16 years later. It's exactly the same recording. There's also an 11 year gap between the last two records.

Amebix 24 years gap (1987-2011)
At The Gates 19 years gap (1995-2014)
Atheist 17 years gap (1993-2010)
Carcass 17 years gap (1996-2013)
Dream Death 26 years gap (1987-2013)
Druadan Forest 17 years gap (1999-2016)
Exumer 25 years gap (1987-2012)
Flower Travellin' Band 35 years gap (1973-2008)
Fuck Off 23 years gap (1990-2013)
In The Woods... 17 years gap (1999-2016)
Massacre 18 years gap (1996-2014)
Mysticum 18 years gap (1996-2014)
Phlebotomized 21 years gap (1997-2018)
Possessed 32 years gap (1987-2019)
Shub-Niggurath 14 years gap (1997-2011)
Siren 21 years gap (1989-2020)
Terrorizer 17 years gap (1989-2006)
Witchfinder General 25 years gap (1983-2008)

Atrophy just pulled off a 34 year gap between albums.

Last year's Heavy Load set the mark at 40.

And it's currently been 14 years since Jupiter, too.

Sweden's Sorcerer made two demos (1989,1992) and came back with an album 2015.

Razor 25 years
Acid Reign 29 years
Hydra Vein 33 years

Toxik (USA) were inactive for quite some time, as they quit in 1992. They reactivared in 2007.

Albums were put out in 1989 and then next time in 2020.

from the finnish osdm scene convulse, purtenance, and abhorrence all had about 20 years of downtime - whether demilich or xysma count depends on how much activity is acceptable

Sorcery (Swedish death metal band)

Released their debut in 1991, and split in 1997 after only one more demo had been recorded (in 1992). Reformed in 2009, and released their next album in 2013. So over 20 years between the two albums.

Whats going on there? I remember talk a few years ago that they had a couple of songs ready for a new album but silence since then

Pentagram is a big one. They had periods of activity and inactivity through the '70s and early '80s where they recorded a lot of the material later found on the two First Daze Here compilations. Then they finally had a band merger between Pentagram and Death Row but Bobby forced the name change to Pentagram. They then released Relentless in 1985, 13 years after their first release. It's more of an example of they basically took over a decade before releasing a full length album, and that full-length album was released a few years earlier as Death Row's "All Your Sins" demo. The first "Pentagram" album where they released an album that was actually only released under the "Pentagram" name was Day of Reckoning in 1987.

And they've been on fire since reforming.

Sarcofagus took some time. Their last album came in -82 (if you count Kimmo Kuusniemi Band as Sarcofagus, which it basically was), before they returned with an album in -07. The "Motorbirds" album (-04) was just a re-release.

Dawn has skipped 26 years now. I believe they can reach the third decade.

EDIT: Okay, this goes against the OP but you gotta admit 26 years is pretty stupid, especially considering the constant "progress updates".

Indeed, everything is great about them but the guitar playing is insane! It's almost like Cacophony reincarnated.

Cloven Hoof and Paradox dipped during the whole of the 90s.

...and Oceans would fit this thread

Tarkus waited 36 years.

https://www.metal-archives.com/bands/Tarkus/3540418284

Yeah I heard there were some songs done but that seems like it was a while ago. I guess we'll just keep waiting.

I'd say festivals like Keep it True or Up the Hammers are responsible for a good 60-70% of bands who make an unexpected comeback 10/20/30 years later...

Most bands who attempt a comeback after such a long break tend to fail miserably, or just turn out ok/passage stuff similar to what they did, decades ago, prior to the last split-up.

A few bands, that come to my mind, who managed to come back AND release absolutely solid, quality, memorable records:
Carcass
...and Oceans

Pretty good, but not quite as solid as early material:
Cynic
Arcturus
Gorguts (I'm personnally not a huge fan of their later material, but there is no denying that the quality is still there)

These are the few names I could think of by doing a quick search. There's probably more, but I still feel like these are the exception and not the rule.

Every one I can think of is an A+. Heavy Load, Psychotic Waltz, Glacier, Hell, Sorcerer, Pagan Altar... gold.

I'm not really all that familiar with these bands and their discograpy, so I wouldn't know. Still, it's not impossible to make a good comeback, but most bands who made a comeback after more then a decade of absence are making really forgettable music since they've reformed.

Sir Lord Baltimore had 35 years between drinks
1971 - Sir Lord Baltimore
2006 - Sir Lord Baltimore III Raw

As a huge fan of the old ...and Oceans stuff, I was quite anxious about them "reuniting" (maybe not really the right word in this case), but did they prove me wrong. Their new two releases are as if they never took a break.

I think VON was not mentioned here yet.

No need to begin with. Venien's Von is fake as fuck despite the short Goat cameo.

Toadliquor just dropped their first album in like 30 years

Some obvious ones that come to mind- Autopsy, Elixir, Brocas Helm, Pagan Altar, Satan (UK), Imprecation, Interment, Ashbury, Manilla Road, Helstar, Attacker, Blood Farmers, Cauldron Born, Destructor, Rekuiem, Witchslayer, Pentagram Chile, Sarcasm...

Toranaga from England.

Their 2nd album 'God's Gift' got released in 1990, and the band split up two years later. They made a comeback in 2010, and released 'Cynical Eyes' EP next year. Plus also an album in 2013, but now it's over a decade again in release department...

Heathen, the best Thrash Metal band, had 19 years between LP's.

Damnation is a fun example of this, 1994 Bathory worship followed by 2004 Bathory worship. Vocalist also did the bass work for Dismember, including on Like an Everflowing Stream, which should give an indicator for the high quality.

https://www.metal-archives.com/bands/Damnation/14594

Celtic Frost - 16 years between Vanity/Nemesis (1990) and possibly the best album of all time, Monotheist (2006). Triptykon are almost 10 years since the release of their second album, Melana Chasmata (April 2014).

I'll add to the answers of my own question: Exhorder, Vio-lence, Mordred, and Morgoth.

Evil is another one! And Zed Yago!

Celtic Frost counts, but Triptykon has played live shows regularly since Melana Chasmata and released Requiem in 2020.