For almost two years, I've been looking for an anime with a group of bad guys who a group of good guys, led by a young man, are trying to turn into good guys by either convincing them or fighting them (I don't remember which). Each bad guy owns an amulet in the shape of a circle, added as an image below, probably, brown-like colored near their (his/her) neck. If they are successfully turned, the amulet decays. Otherwise, IIRC, they transform to trees or tree-like things.
The amulet look like this sketch:
I watched it by 2005. However, the anime's production is highly likely prior to 2005. I watched it on TV, dubbed in Turkish, before going to school.