Bohrok-Kal

Bohrok-Kal were elite Bohrok that were released on Mata Nui for one purpose: free the Bahrag.

History
Shortly after the Bahrag imprisoning, six mutated Bohrok called Bohrok-Kal were released. These elite Bohrok were much more powerful and intelligent, they could even talk, but they were more independent than regular Bohrok. Their mission was to release the Bahrag from where they had been imprisoned by the Toa Mata.

Immediately after imprisoning the Bahrag, the Toa were transformed by Energized Protodermis into more powerful Toa Nuva, and each gained a Nuva Symbol that was tied to their elemental powers. The Bohrok-Kal discovered that the Bahrag's prison was locked by a special Nuva Cube, which required the Nuva Symbols to unlock. The Bohrok-Kal had to steal the Symbols and find the sealed entrance to the Bahrag's underground lair and prison before actually releasing the twin queens.

And indeed, the Kal managed to steal the Symbols easily, find the lair's entrance, clear a way in, and almost finish their devious task. In the lair, a group of Exo-Toa were guarding the prison; the Bohrok-Kal defeated them easily but it gave the Toa Nuva time to catch up to them. Toa Tahu tried to use the Vahi,the Great Mask of Time, against them. However, even slowed to a near-standstill, the Bohrok-Kal were protected by their silver Krana-Kal. In a last-ditch effort, the Toa banked on their ties to their Nuva Symbols and fed their strength to the Bohrok-Kal, giving them power beyond imagining - too much power for them to control. Their bodies were utterly destroyed by their own powers and only their Krana-Kal remained.

Powers
"Let's just hope there aren't any swarms of them somewhere. These six look to be enough to deal with"

-Gali Nuva refering to the 6 Bohrok-Kal

The Bohrok-Kal (minus Lehvak-Kal) had some of the same elements as normal Toa. (Electricity, Magnetism, Sonics, Plasma and Gravity). They also used a set of enhanced Krana called Krana-Kal that have slightly different powers; and when the Bohrok-Kal were in the final moments before completing their mission, their Krana-Kal turned silver and projected a shield that protected them from harm.

The Bohrok-Kal also had shown the ability to merge into Kaita beings (taking the name of whichever Krana was dominant), which combined and amplified the powers of those who took part in the fusion. Bohrok and Bohrok Va also had this ability.