Krana-Kal were specialized Krana, mutated by the Bahrag's energy and a mutagenic substance. Like regular Krana that were literally the brains of the Bohrok, the Bohrok-Kal were controlled by the Krana-Kal. The Krana-Kal were unlike the other Krana in that they could act independently, without depending on the Bahrag, and were able to communicate with other beings using telepathy. Krana-Kal also had a unique property that allowed them to turn from a different metallic/opalescent color to a high polish silver color and generate an impenetrable shield around their hosts when at the final stages of their mission.
The Toa Nuva defeated the Bohrok-Kal by overloading their powers with the Nuva Symbols. The Bohrok-Kal were destroyed, with only the Krana-Kal remaining intact.