Dark El Nacho is the evil alter ego of the great omnipitent being, El Nacho. The creation of this secret split- personality came about when El Nacho was fighting his mortal enemy, the Jasselasaurus, and was covered from head to toe in the most evil element, Fluoride, which would usually kill most people, but simply caused this strange anomaly.
When El Nacho feels it is neccessary, he can tap into his Dark side, which gives him power equal to that of the square root of negative one to the power of infinity. However, this comes at a terrible cost, as it causes him to gain an insatiable bloodlust, causing him to attack anyone who comes near him, (hence the name "Dark El Nacho", and not "happy fun clown puppy-dog loving El Nacho".) Due to this, he avoids the use of this awesome power where possible.
Another reason for not tapping into this Dark power is the fact that Dark El Nacho always orders anchovies on his pizza, and no-one likes anchovies.