The Megalith is an ancient creature that feeds on energy from the realm. Binding the Megalith was done in cycles, documented as far back as the Bakkorian period in "The Cycle". Nazmul was able to take control of the Megalith by carving it a guardian stone with Sari, and cast a spell that drained a lot of the realm's sigil energy to feed it to the Megalith.
The Pure Stones and the Machine of Binding were a new development by the Wizards of Yan. This binding not only sealed the Megalith, but the Ethos as well. But because of the damage it caused to RaDos each time the Spell of Binding was cast, it shattered a large portion of the realm and sacrificed the Defenders' parents. As the years went by, it slowly began to break through the seal.
The Defenders managed to get their hands on all four Pure Stones, but were warned previous times by Sari and Brim about just how devastating the spell was. And they didn't realize it until the Spell Zone was being torn apart and Nazmul explained it to Seth during their battle. Although it would cause the Megalith to be sealed forever, it would have completely destroyed RaDos.
They stopped the Spell of Binding by destroying the machine before it was complete. The Megalith breaks free of its prison, planning to regenerate itself from the sigil energy of Arboth. During its rampage, it possessed an old woman to distract the Defenders. In the process of battling her, it separated Kragus from Seth. When they reach Arboth, the Defenders along with Adam try to keep the beast under control with everything they had.
But not even Omnikrag, freezing, or incapacitating it wouldn't work. Utilizing the power of the Warrior Henge (between Seth, Mel, Kara and Adam), the Nova Stone, and Kara's power of energy absorption, the Defenders defeated the beast, giving the sigil energy back to RaDos.