So you may wonder what a darkstone toa is and even if you aren't wondering and don't care, I'll explain anyway.
Darkstone toa is a toa that has become corrupted by shadow protodermis (not sure if this is actually in the bionicle story, but I made it up all the same) and mutated into darker versions of their former selves.
For this guy, as an example, he has been mutated into a more skeleton-like creature. He carries twin shadow obsidian blades that he can move and swing without even touching. He usually hides in the ground and leaps up to catch whatever come along in his way. He hunts in the shadow mountains and attacks any living thing.
There will be much more darkstone toas in the future.