This creature was somehow made by the mutated Lohrak.
The Toa Metru were able to destroy it by sucking the air out of the entire room, then shooting an ice blast at the being - since there was no air in the room, sound wasn't able to exist either, so the creature could not fight back. It then shattered and melted away.
It was a dark, nebulous creature, made entirely out of sonic force. Whenever it struck something, or something struck it, it triggered a sonic boom.