The Skull Army was a force of villainous, undead beings, under the command of Kulta, the Skull Grinder. Their aim was to find and destroy the Mask of Creation, an order given to them by the former Mask Maker, Makuta.
Thousands of years before the arrival of the Toa, during the time of the Mask Makers, the island of Okoto was plagues by invaders known as the Skull Raiders. The Skull Raiders were eventually repelled underground by the Mask Maker Ekimu and his brother, Makuta, though Makuta secretly betrayed Ekimu and forged an alliance with the Raiders to form an Army when he next contacted.
Many years later, Makuta's treachery was exposed and he was defeated in the Battle of the Mask Makers. His dark spirit was eventually able to return to the island and contacted the Skull Raiders leader, Kulta. Kulta left his underground confinement and began raising an army of Skull Creatures.
Battle for the Ancient City At the command of the Lord of Skull Spiders - under Makuta's control, Skull Spiders attacked the villages of Okoto, seeking out the Masks of Power. The Toa were summoned down by the Protectors, and seeked their own Golden Masks. Skull Spiders trailed the Toa but were defeated.
In the Ancient City of the Mask Makers, Kulta used his dark powers, granted to him by Makuta, to raise an army of the undead, consisting of Skull Warriors, Skull Scorpios and Skull Slicer, with his old lieutenant Skull Basher, a former Skull Raider, joining the army. Kulta placed them as guards and sentries, whilst claiming the Mask of Creation and taking it to the Forge to melt it.
The Toa arrived at the city and encountered the Lord of Skull Spiders, working together they defeated him. The Toa proceeded to the Gate and were met with a group of Skull Warriors. They were easily dispatched. They passed through the gate, Lewa broke off from the group and encountered Skull Slicer, who stole his mask. The other Toa found him and defeated Skull Slicer, causing the Arena floor to fall.
They escaped the rubble into a Graveyard where they encountered a group of Skull Scorpios, though they stole Pohatu's mask, the Toa defeated them too. In the Graveyard they found Ekimu's tomb and awoke him.
With him they headed for the Forge but were blocked by Skull Basher. He stole Onua's mask but working together, the Toa blasted past him. In the Forge Kulta donned the Mask of Creation and fought the Toa, whilst Ekimu reassembled his Hammer of Power. Ekimu then surprised Kulta and removed the mask from his face.
Kulta and Skull Basher were then imprisoned, officially ending the Skull Army.
The Skull Army was composed under Kulta's leadership, a fellow Skull Creature. The transparent red armour on the members was a sign that they were under his command. Kulta in turn was acting on the orders of the Mask Hoarder, Makuta, who was also direction the Lord of Skull Spiders.