Noob Moron lokking for anime from childhood

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Hello, all. I hereby subject myself to the hostility that I may incur as direct result of this post.

I have checked the old toonami logs for animes aired from 2002-2006, but have been unable to find anything which jogged my memory.

The anime had a very dark theme throughout. Much of the story took place at night, in dark forests, etc. The main character was seemingly in his teens, and had (maybe) slick black hair. He was often accompanied by a very old, small woman, possibly his grandmother, who was also a sage or expert on magic/mysticism. There was much fighting, and it was one-to-one if I'm not mistaken (Think pokemon or yugioh, not One Punch or Seven Deadly Sins - as far as fight scenes go) The fighters were almost certainly magical, but all human if I remember correctly.

Here is the scene I most vividly remember: Main character is slumped against a tree in a moonlit forest. the old woman is near him, talking passionately, perhaps egging him on so that he fully commits to whatever is about to ensue. The boy begins to writhe in pain, continually more intensely. (I'm not sure how it fits in, but I am almost certain another key character in the scene is a small unassuming and non-threatening looking creature similar to a ferbie, though this may be false) the intensity of the scene grows, and the boy begins to eat himself inside out. End memory.

I understand that this post is almost uselessly vague but please help if you can. I have had this cliffhanger in my mind for years and it absolutely drives me nuts. Thanks.