Hakumen no Mono from Ushio and Tora
Many people might be surprised by this one, but I knew that I would like him even before I saw him on screen. This is a character that understands how humanity works and uses long running plans to gain damage the side of humanity. These plans are using the fear of human kind to make them turn against each other. From manipulating mankind’s military to attack false targets to planting false weapons amongst those with spiritual awareness and even splitting the forces of demon kind by making the East and West demons fight against each other, he is a master mind. And noticed I said before he showed up.
When Hakumen no Mono finally makes an appearance, the sense of how doomed humanity is all over. Just by leaving his resting spot, he is already making Japan sink into the ocean. Hakumen is a big and scary form of pure evil that only grows more powerful the more humanity fears him. Having a sinking island nation nearby helps with that. In the end, it took quite a lot of costly sacrifices to bring him down.