This is the fastest I've ever made one of these concepts... yay.
This pitch brings back the idea of taboo masks.
This specific mask, the Mask of Monsters, is one of the few mask a Matoran could wear and use its effects... at a price. The user mutates and gains size, strength, speed and a random set of powers (and maybe a couple of limbs) becoming a Rahi-like monster. The problem is that the users essentially Hulk out and become mindless beasts -hence the taboo status- until the mask is forcibly removed from them.
What makes the Rahkshi special is their connection to Makuta and the Mask of Control. This lets them (mostly) keep their intelligence and consience, making them extremely formidable, as well as hide in plain sight if they choose to remove the mask. As per Bionicle tropes, the Cult of Makuta have six.
As for the piece, there is only one mold for the mask, but have four holes on the top to allow connections for horns. They use the same connections that I believe some of the Disney minifig bags use (two examples I can find are this one for Donald Duck with his hat and Aladdin's genie with his hair) and could be arranged in different positions or use different horn pieces depending on the Rahkshi.
So yeah, that's pretty much it.