Actually there is no clear line between avant-garde and experimental metal(or transcendental metal) so i will keep them in mind as one genre.
I think it is importent to listen it, metal have so much potentional, that we can use in IDM(for example igorrr) and avant-garde metal stay close to all experimental music. I have some bands for your start
Liturgy - transcendental black metal project by composer Haela Ravenna Hunt-Hendrix. This band has gone through evolution from Renihilation with black metal arrangement and noisy sound to 93696 with symphonic and christianian musical motifs. But one thing remains the same - special drum technique). They called it burst beat, you definitely need to listen that.
Favourite albums are H.A.Q.Q.(especially HAJJ, VIRGINITY and PASAQALIA), Aesthethica(Generation and Veins of God just cool instrumental tracks and Returner, Tragic Laurel and Sun of Light with special Liturgy black metal guitar and drums style) 93696 - you need to listen it full
Also Greg Fox is the past drummer from Liturgy, he had some interesting music if you interested. He played in band Ex Eye - metal, jazz, avant-garde masterpiece
Zeal & Ardor - a Swiss avant-garde meta band with soul influence. Conceptual basis for the project: What if American slaves had embraced Satan instead of Jesus? The main feature of the group is the vocals of Manuel Gagneux.
Favourite albums are Wake of a Nation - BLM black metal with a very good sound. Like it as a whole, Zeal & Ardor(2022) - album with some remakes of past tracks. Golden Liar, Erase, Bow and Feed the Machine are the best.
Peste Noire - avant-garde nazi black metal. If you can’t listen to politically incorrect music, then skip ahead. That band have a special black metal guitar style(like psychodelic rock or something like that) and marching rythm(not like march in power metal, but close to medieval one) Ballade Cuntre Lo Anemi Francor - my first ever listen experimental black metal album. It come with raw black metal sound and melancholy atmosphere. Ballade Cuntre Les Anemis De La France, A La Mortaille ! and Rance Black Metal de France are the best metal songs in the album.
Botanist - green floristic avant-grass metal. Black metal arrangement sometimes without traditional distortion guitar sound. He uses Hammered dulcimer. Very unique and beutiful band
The favourite albums are I: The Suicide Tree / II: A Rose From the Dead - black metal without guitar, you need to listen it VII: Beast of Arpocalyx - long tracks and distorted dulcimer.
Bliss Signal - experimental blackgaze with high techno influence and extremely melancholy atmosphere. 808 kick blastbeats and noisy textures go together perfectly with black metal
They has only one album Bliss Signal…so listen it
Deathspell Omega - avant-garde black metal band with big discography. They had a special dissonant guitar sound, which makes the band very unique and evil.
Favourite albums are Si Monvmentvm Reqvires, Circvmspice (First Prayer and Si Monvmentvm Reqvires, Circvmspice are favourite) Paracletus - very dissonant and interesting (Wing of Predation, Abcission, Apokatastasis PantĂ´n - the best)
If you have some another favourite avant-garde metal, so tell us about them.
I know about 40-50 avant-garde metal, so if you need more bands - write below
They way I see avant metal is it’s pretty much this vague mixture of death/black with symphonic elements and prog tendencies, usually fairly artsy and stuff, but there’s a whole lot of interesting bands. I liked Unexpect back in the day, there’s also this little band Carnival in Coal that’s kind of fun.
Ages Gone - S/T
Really odd German Death Metal, self - proclaiming themselves as Free Death. There are synths that are not influenced by black metal, and kind of give it an enjoyable amount of cheese at times. The vocals are pretty well done, and the album experiments with turntables and samples later in the album.
Demilich - Nespithe
A Technical Death Metal classic, it also has really strange and angular riffs. Pretty heavy overall and an interesting take on the genre. Also has the lowest vocals I’ve ever heard.
Disharmonic Orchestra - Not to be Undimensional Conscious
My personal favorite, it just has the most ideas that should not work but do so well. Clean and prominent bass. A guitar with overdrive distortion. Vocals that are almost growling, but not quite, along with surrealist lyrics. Technical and very groovy songwriting. Yet it all works so well I haven’t really heard another album like it.
Effluence - Pianistic Dismemberment
Off the walls Brutal Death Grindcore is the only way i can describe it. The brutality of Brutal Death Metal, along with the insanity of Grindcore. also NO GUTAIRS, CUS PIANO ATE THEM :[
Not the biggest fan but its a solid one time listen if anything. Theres some real creative stuff going on there.
Not sure if bands with progressive metal influences fit here, so for right now thats kind of it.
I could get into Pleasuredome by Disharmonic Orchestra but i need to give it more listens before I come to a final opinion.
Some another good stuff that i remembered
7. Victory Over the Sun - avant-garde blackened death metal band. Dirty sounds and emotional guitar chords.
Favourite albums are: Nowherer - written entirely in the 17 equal divisions of the octave (Nowherer, Alveromancy) Światło w chmurze (released under the name Cichy Duch) - that album is felt like mix of raw DSBM and Liturgy guitar parts (Światło w chmurze, Wzory i figury)
8. Circus Satanae - the funniest avant-garde blackened death metal/grindcore i ever listened. Also can be described like not evil black metal Nuclear Moments… - very nice album, i like all these songs.
9. Pensees Nocturnes - circus avant-garde black metal. Very stylish and qualitatively. Feels like circus in Hell
Favourite albums are GRAND GUIGNOL ORCHESTRA(Deux bals dans la tête, Poil de Lune, L’Alpha Mal) - very dissonant black and really beatiful orchestral arrangement DOUCE FANGE(Quel sale Bourreau, Quel sale Bourreau)
10. Grey Aura - avant-garde black metal with interesting guitar parts Zwart vierkant( Maria Segovia, Rookslierten, flessen, Parijs is een portaal)
11. Igorrr - avant-garde baroque breakcore metal. My favourite band, i love absolutely full discography(especially Spirituality and Distortion and Savage Sinusoid). That’s a really great masterpiece (and i think that you already have listened them)
12. Oranssi Pazuzu - psychedelic black metal band(but also i’ve seen tag like transcendental black) Really untypical(like everyone in this list ) project where every album create unique style and atmosphere Mestarin kynsi(Uusi teknokratia, Kuulen ääniä maan alta) - great album with my favourite track Kuulen ääniä maan alta. This track opened my eyes on connection between black metal and electronic metal first time Kosmonument(Komeetta, Maavaltimo) - early album where a connection can be traced between psychedelic rock and black metal
) project where every album create unique style and atmosphere