I already posted this in the BIONICLE 2017 Topic, but I added something which could apply to this topic.
What if the drained parts of Umarak's hunter mask... were drained by Makuta so he could have the power of shadows in the Mask of Ultimate Power?
Think about it.
We might assume that there were only the six elements shown to us to be forged into the Mask of Ultimate Power.
"It was a sacred law that a mask could never contain the power of more than one element, otherwise it would become to strong and dangerous." -The Legend
Now it never specifically says, "It was a sacred law that a mask could never contain the all the power of all six elements, otherwise it would become to strong and dangerous."
"A mask could never contain the power of one element" compared to "A mask could never contain the power of all the elements."
This line could imply that there are more elements than just the main six.
Shadow could be one of them.
That would explain the evil eyes of the Mask of Ultimate Power. A dark element was placed in a mask with the six foundation elements of Okoto.
Umarak was born from the shadows when Okoto was first beginning. This implies that shadow is an evil element.
Not to mention, the Mask of Ultimate Power 'Shadow-Flames' are similar to Umarak's shadow powers in BIONICLE: Journey to One.
Then I had another thought...
In the animation Fallen Heroes, it shows Makuta with the Mask of Control, but there was a flame around it.
Could the flame represent the shadow-flame of the Mask of Ultimate Power (notice the background between the two)?
But shadows are not really associated with the G2 Makuta so... Makuta Teridax of G1 perhaps?
In that same episode, Pohatu said, "Under the evil Makuta's control."
The G2 Mask Maker? Or the G1 Teridax?
In other words, the Toa know that Makuta is behind the evil, but they do not realize that there is another Makuta.
So they're right... and yet so dead wrong.