Skull Creatures

The Skull Army is the group of the undead beings who served as the major antagonists in the 2015 storyline and later the supporting antagonists in the 2016 storyline in the BIONICLE series. They are also led by Kulta and later by the true leader of the army, Makuta the Mask Hoarder as he was responsible of leading them to defeat his brother, Ekimu and conquer Okoto.

They are also formerly known as the Skull Raiders who was responsible for plaguing Okoto before the start of the series.

Early Life
Thousands of years prior to the start of the main series, during the times of the Mask Makers in the planet of Okoto, the group of ancient pirates called the Skull Raiders, who were led by their leader Kulta, invaded Okoto. However, they were opposed by the two Mask Makers, Ekimu and Makuta the Mask Hoarder, though the latter who was consumed with greed and jealousy secretly betrayed his brother as he approached towards the Raiders after they were defeated and driven into the underground city in the mountains and forged the alliance with them to form an army.

Many years later following Makuta's defeat in the hands of Ekimu, Kulta and the Skull Raiders were trapped in the underground city and as the result, they were buried alive as they were transformed into the undead beings.

Quest in Okoto
After quite sometimes later, the Skull Raiders were remained being buried underground until the arrival of the Toa Mata, who later transformed into the Masters