Ok, so the Mask of Time is the most powerful mask in existence. I can see that.
It takes all the Brothers' power to make this specific mask, but here's something that bothers me: if it takes the power of all three to make it, and only one of the three can wield it at a time (which means whichever of the Brothers wears it automatically becomes the strongest):
How would the Toa defeat them (excluding Karzahni)?
According to what you said, the Mask of Time has almost all the raw power of the Three Brothers imbued into it. That means the Mask of Time alone is almost as strong, if not equally strong, as the Three Brothers.
I'm assuming the Brothers regenerated their power after the mask was created, meaning the Three Brothers+Mask of Time would be worth roughly six Brothers.
If even one of the Brothers has access to the Mask of Time, they should automatically win (via time manipulation, slow motion, etc.). Did they lose it at some point? Or did the Toa take the mask and use it to defeat them? If so, how did the Toa take it?
However: I have one idea as to how the Three Brothers were defeated, step-by-step (I know Karzahni was out of the picture, but still):
- The power of the Three Brothers is tied directly to the Mask of Time after it is created. If the Mask is shattered, the Brothers are defeated.
- The First Toa are created and told how important the Mask of Time is by the Elemental Gods (Nuva).
- The Mask is stolen by the Toa (somehow).
- The Mask is shattered, severing the Brothers and their source of power.
- The Three Brothers are defeated.
Here's how Makuta could be revived:
- The Rahkshi gather the fragments and freeze the island (mutation follows). Power begins to slowly return to Makuta.
- The Toa are summoned.
- Hijinks ensues.
- The Rahkshi are beaten.
- The Toa release all the power from the Mask of Time, undoing all its effects and destroying it forever.
- Makuta regains all of his power.