The talent "Oathbreaker" belongs to the Dark Series of Priest talents. Although the description of the talent is vague, players are well aware of its effect, which is to convert the player's Light Damage attribute into Dark Damage.
Back in the level 60 version of the game, if players wanted to unlock this talent, they needed to complete a specific class quest called "What Brightness Won't Tell You." Only after finishing this quest could they activate this talent, as well as the unique Dark Priest skill "Dark Form."
Only by stacking various Holy Series damage-boosting talents could Little Black's build (BD) be achieved.
Up to now, aside from this build, there are hardly any other builds that would pick this talent. The reason is quite simple: everyone thinks it merely converts Light Damage into Dark Damage and doesn't help with other builds.
Moreover, this talent itself takes up a lot of talent points.