Irnakk

"You use fear as a weapon, the same as I. I make you fear life, and in return, you make me fear death. You have found your true being - your essence is darkness and the grave, Piraka. The pit yawns for you, and who am I to keep you and yours from it?"

- Irnakk to Zaktan, just before disappearing

"As you know, all of the beings now known as Piraka are from the same species and originated on the same Island (though they are scattered about in many places these days). As with any culture, they had their share of myths -- figures of good, figures of evil, figures that might exist and figures that couldn't and shouldn't.

"Irnakk falls into the category of 'things that should not' --he's a figure of nightmare, tales of which the rulers of that species' island use to keep their people in line. In the same way the Turaga used tales of the Toa to fill their people with hope, the rulers of this species used tales of Irnakk to fill their people with terror.

"When the Piraka get closer to the Kanohi Ignika, they encounter an area in which this figure is drawn from their nightmares and made real. The Piraka, beings who inspire fear, must face their own horror if they want to reach the mask"

History
Irnakk was not a real being, but it was a myth. The leaders of Zakaz told the rest of the Skakdi about Irnakk to keep them in line.

Later, when the Piraka entered the Zone of Nightmares, Irnakk appeared and made Hakann's armor melt. Zaktan became a mere thought, Avak was trapped in a prison (his own body), Thok was attacked by his own animated creations, and Reidak and Vezok were stuck in a never-ending battle.

Eventually, though, Zaktan was the one who conquered his fear and defeated Irnakk. After that, Irnakk simply disappeared.