The Dark One is the leader of the Dark army. Cerrax described the Dark One as "the Neosaur that was filled with greed, jealousy and hate".
There were a lot of storyline events involving the Dark One.
Cerrax was the Dark One's mortal enemy. The Dark One was always jealous of him and he wanted to be leader, too, yet the Saurians never voted him as leader. The Dark One was mad and started an army of his own. He took over Saurian races and encouraged them to fight for "the bad side". The Dark One felt good about his actions. Cerrax knew about this and he gathered his army. Cerrax's army and the Dark One's army fought. This was then called the Great War. Cerrax and his army won. "The bad side" has lost, but the nature of Inner Earth was lost, too. The plants became dry and the environment looked bad. Time went by, and the Inner Earth's nature was restored.
The Dark One is also the cause for Dr. Leonid's disappearance. The Dark One ordered his soldiers to kidnap Dr. Leonid, thinking that it would be a good message to send to Cerrax, as Cerrax's daughter, Didi will see everything.The Dark One is also the cause for the birth of Dark monsters. The Dark One influenced these monsters to be on his side and fight against the good side. As for now, there are Dark monsters all over Inner Earth, and they are often proven by their name being written in purple and their ability to overpower its non-dark counterparts.