Krana-Kal were living creatures which resided inside and operated the Bohrok-Kal mechanoids. As with the Krana, there were eight types of Krana-Kal: Krana Bo-Kal, Krana Ca-Kal, Krana Ja-Kal, Krana Su-Kal, Krana Vu-Kal, Krana Xa-Kal, Krana Yo-Kal, and Krana Za-Kal. Each provided the Bohrok-Kal with a different power or ability. Unlike the Krana, the Krana-Kal could function independently of the Bahrag's influence and communicate with members of other species. If a Bohrok-Kal's Krana-Kal was removed, the mechanoid ceased to function properly. At least some of the Krana-Kal escaped the destruction of the Bohrok-Kal.
The krana-kal were the driving force behind the Bohrok-Kal threat. They were intelligent, powerful, and able to plan strategy and communicate with other species. They brought a new dimension of danger to Mata Nui. Although there were only six Bohrok-Kal, each possessed eight krana-kal. Some of the krana-kal were so specialized that they were only of use at certain times and in certain places. Replacement krana-kal were carried by the Bohrok Va, who served the Bohrok-Kal.
Along with all their incredible might, the Bohrok-Kal hid a secret that could spell disaster for the Toa Nuva and Mata Nui. When the opportunity to free the Bahrag was within their reach, the krana-kal transformed to silver and wove a field of energy around the Bohrok-Kal. This force field protected them from any physical harm from outside, ensuring that nothing could stop their mission.
Like the Bohrok, each of the Bohrok-Kal carried within it a krana-kal. The krana-kal were living creatures capable of independent thought and communication with other species.