The Kanohi Vahi was the Great Mask of Time. It was able either to slow down or speed up time around a target. It was originally created by Toa Vakama using all six of the Great Disks.1
Notes
The Kanohi Vahi was the Great Mask of Time. The wearer of this mask was able to slow the passage of time, in order to move faster than his opponents.2
Even the Turaga feared the Mask of Time.3
The power of the Vahi was so awesome that it could mean the end of all things. Vakama warned that its power might be too great for even a Toa to wield. Tahu Nuva was barely able to control its power for a short time.4
The Kanohi Vahi was the Great Mask of Time. It was the most powerful mask of them all. Its surface gleamed a deep flame orange. Turaga Vakama warned Tahu Nuva to think of the greater reality when using this mask, as he who controls time controls reality. He warned that it was one thing to control time and all of reality with it, and another to lose control of it all. He said the Vahi could only be used in the direst emergency, when there was nothing to lose. The fiery orange mask had an odd, primitive shape. The mask glowed when activated. When used, a ripple of energy would radiate out from the mask, and the air would seem to quiver as time energy moved through it. Tahu quivered uncontrollably while using the mask, barely able to control its power.5
Vakama crafted the Mask of Time from the Great Disks.6
When Toa Vakama used the Mask of Time, a wave of temporal force flowed from it, striking Makuta. It slowed the movements of Makuta and a stream of energy coming from him almost to a standstill. However, Vakama's control of the Mask of Time was imperfect. He was unable to stop the same temporal wave that slowed Makuta from slowing himself as well, which meant that he could not avoid Makuta's attack.6
The Vahi was considered a Legendary Mask.7, 8
It was created from the Disk of Time, which was made by combining all six Great Disks.9
The Kanohi Vahi, the Mask of Time, was created by Toa Vakama. It could slow down or speed up time around a target. Its destruction would rip time itself to shreds. The Vahi was used twice: once by Vakama against Makuta Teridax, and once by Tahu Nuva against the Bohrok-Kal. At the end of story year 2008, it was on Metru Nui.10
The destruction of the Vahi would only have affected the Matoran universe, not the world outside of it.11
As of the ending of the story, the Vahi was on Spherus Magna with Voporak.12
The Vahi did not turn grey when not worn, and it did not change color when worn by a Toa. It always remained orange.13
The Vahi could not completely stop time, but it could slow it down considerably.14
Losing control of the Vahi while using it could have disastrous consequences on reality, so it was not to be used lightly.15
The Vahi couldn't cancel out Voporak's aging power because it could only speed up or slow down time around a target.16
A broken Vahi would not banish time, it would banish orderly time.17
The Vahi was one of very few masks which did not turn gray when not being worn.18
The Kanohi Ignika was not more powerful than the Vahi, they just had different powers.19
The Ignika and Vahi were on the same power level because the elements they represented were codependent.20
The Ignika and Vahi didn't necessarily have different levels of power, just different kinds of power. Life couldn't exist without time, and time was irrelevant without life to affect.21
The Vahi could speed up or slow down time around a particular target. No known being could make it affect the entire universe.22
There was only one way to make a Vahi.23
The Vahi could be used to slow the Ignika countdown.24
Other Information
Greg Farshtey was completely opposed to introducing time travel. He felt the story would be over as soon as it was introduced unless the past couldn't be altered, but that would render time travel pointless.25