There are some albums that combine different eras of a band by being written in one time period and then finished and released in another. Examples of this include Morbid Saint - Destruction System which was written in the early 90s and released in 2015, Darkthrone's "Goatlord" (1996) which would have sounded different had it been finished around the time it was conceived (after Soulside Journey in 1991) and Hell - Human remains, with songs first put on demo in 1982 and then finally released in 2011 with an extremely modern sound and an allover approach to recording that would not have been possible in the 80s.

Since I like all three of those examples I thought this might be a trait to look out for and maybe you know of other albums that took long to finish and, because of this fact, mixes multiple stages of a band's history.

Fear Factory - Concrete album, written long before the official release

apart from hell's human remains, which was already mentioned in the op, i can only think of morbid angel's abominations (rather smaller scale "time travel")

Inception by Sanctuary apparently contains recordings from 1986. It sticks out a mile from the band's comeback album.

This was released as it was recorded, I don't think it was amended or altered in any way before it was issued. Not sure that's what the OP is looking for.

oops, I misunderstood

The new Pagan Alter I think?

All of their albums are. They wrote their first three in the 70s and 80s, and released them much later as proper albums. The new one is their most recent material, but written 13 years ago and released this year. Pretty interesting to think of the level of songwriting and atmosphere they've always had - what a talented band.

I know it isn't what OP is looking for but upon reading the title, I immediately thought of Nocturnus - The Key

"Planet Satan" sounds like it was released immediately after Mysticum's first and not eighteen years later. Doesn't mix different sounds of the band, but rather sounds like the band themselves stepped out of a time portal and missed the entire 00's!

Scum recorded Garden of Shadows in 1996 but didn't release it until 2016. This one only sort of counts I guess because the band split up after they recorded this album. There is no modern era of the band to compare it to although I think it's safe to say that they'd sound at least a little bit different. Garden of Shadows was a nice time capsule showcasing OSDM because it's the real deal. It's not a retro release or a "return to form" for an old band. It just real OSDM released way later than intended.

The entirety of Pentagram's career is time traveling. Aside from the First Daze Here compilations, every Pentagram album has had songs that were written in the early to mid 70s. There is also a case where the debut album was recorded as a demo in 1982 and released in 1985.

Lunar Aurora's Elixir of Sorrow and Zyklus were apparently recorded much earlier than their 2004 releases. (I'd highly advise checking out the latter, it's right up with Vikingligr Veldi in terms of classic atmospheric bm.)

Bathory's Blood on Ice was mostly written in 1989, wasn't finished and released untill 1996.

Pentagram's First Daze Here i think ...

I heard there are some well bad recorded demos from that time. The tracks were re-recorded and released in 1996 by what i heard about.

Not to mention that they do have a release called The Time Lord. I don't know for certain if it was intended to be, but it sounds like a Dr Who reference to me.

I think Napalm Death's "Scum" qualifies as this, as side A of that record was written and recorded by completely different band members, with the exception of the drummer Mick Harris who played on both sides.

Jesu's Heart Ache/Dethroned compilation from 2010. The Dethroned EP was mostly recorded alongside Heart Ache in 2004 but was never released. Jesu's sound had gone thru all sorts of changes by that point.

Assuming this qualifies, I'm surprised we took this long to mention Manilla Road's Mark of the Beast. I only sort of know the whole story so please correct me if I'm wrong, but from what I understand it was originally written and recorded in 1981 under the intended title of Dreams of Eschaton, the band wasn't happy with how it turned out and basically threw it in the trash and wrote a new album instead. A bootleg of about half the songs floated around for like two decades before Monster Records cleaned up the original masters and released it in 2002 as Mark of the Beast, a full 21 years after it was recorded. I'm not entirely sure if this qualifies since these aren't new recordings of much older songs for the album and are instead just remastered versions of the original recordings from all those years ago, but it's always the first example I think of.

The Mark of the Beast song it's very similar to the Avatar song,which i personally think it's great though.

Thanks everyone for the great suggestions. I love albums and bands with such interesting stories and timelines. Keep em coming if you know more.

Angel Witch's last record was largely written in the early 80's. I know for certain that at least two of the songs on it were live staples from the band hey day.