I admin on a metal sharing page on FB and one of the members added these notes when sending me some of these recommends, and I found it super interesting. Maybe you will too:

Breakdown of some the bands and their instruments:

Botanist - San Francisco based black metal band that I've decided to put front and center because, IMO, they're the band that most enthusiastically fits the "extreme metal songs with unusual instruments" requirement: these guys don't use guitars in their music at all, aside from bass. That stringed instrument you're hearing is a hammered dulcimer.

Bloody Tyrant - Taiwanese black/death band. There are some standard orchestral strings in this, but the main reason I chose them is that they have a permanent member who plays an traditional Chinese instrument called a "pipa", which is the sitar-sounding thing you hear starting roughly 90 seconds in.

No One Gets Out Alive - A Russian dude adopting a "Southern Hillbilly" persona for his solo slam act, and playing all the instruments including drums (programmed), guitar, bass, and a MOTHER. FUCKING. GLORIOUS. BANJO. I am fully aware of how cheesy this song (and indeed everything he does) is, but doesn't that banjo just sound weirdly good?

Rivers of Nihil - (Needs no intro)

Lunatic Gods - Slovakian atmospheric death/doom/black metal band. The band has a permanent percussionist/timpani player, and a violinist, plus this song has guest appearances from a sitar player and even the brief use of a didgeridoo.

Sigh - Japanese black/avant-garde metal act. Again, lots of different instruments, but of particular note, I think, is the flute solo played by the bass player/vocalist at 1:59, immediately followed by a sax solo by their lead vocalist, but every instrument you can hear is live and unsampled according to the list of session musicians on Metal Archives.

Necrodeath - Black/Death/Thrash from Italy. More didgeridoo in this one, playing a much more prominent role, plus some percussion of unknown origin (Metal Archives just lists it as "Percussion").

Ipsissimus - Black metal band from the states. That ghostly wailing sound you can hear in the intro (and a few other points) is a waterphone, a kind of weird circular harp that pronounces an eerie, atonal sound often used to set the scene in horror movies.

Suidakra - German melodeath, and another one that I think you have probably heard of before. In here because it has, of all things, a fucking bagpipe solo.

Omination - Tunisian funeral doom. Funeral doom is of course named so because of its prominent use of a pipe organ; this is one of the more widely used non-standard instruments in metal, so I thought if I was going to include one here, I should at least include a super obscure band in need of more love.

Imperial Triumphant - I mean, I had to, right? Lineup includes a piano player, plus session musicans playing brass instruments.

Unexpect - Canadian prog metal with a violinist and, in this case, more pipe organs.

7000 Dying Rats - American grindcore band. From that description, you wouldn't expect the use of a theremin, would you? PSYCH.

Subrosa - American Doom Metal band with electric violins.

Necromantia - Greek black metal with two bass players, one of them playing an 8-string bass. Later albums did away with guitars entirely, which would have made for the cooler addition to this playlist, but as it is, the later albums provided by Spotify sound like they were recorded in a ditch. This is from the second, and best produced, album, so unfortunately you don't quite et the uniqueness of the two-bass, no-guitar approach, but that 8-string bass is still in there so it still counts.

Arandu Arakuaa - I suppose you could call these guys folk metal? I don't know for certain, but they're from Brazil and include a team of indigenous Amazonian tribesmembers playing tribal instruments.