I was listening to Thergothon last night on YouTube and had a look through the comments below. Someone mentioned that Boň from Russia released a funeral doom demo/ album in 1991 which pre-dates Thergothon and may well be a pioneering recording for that style. I had never heard them before but was naturally intrigued and they sound really good to me. I would say they lean more towards the slower parts of Winter but you can still hear similarities to Thergothon.
Worth a listen.
https://youtu.be/Mu-6aPbJLUw