The Ultimate Power

This is the single most feared thing in The 15 Keys Saga. The Ultimate Power comes from The Ultimate Key, which gives it sentience. However the Key itself is malevolent and does not intend this power to be used for good, as it believes those who are good are undeserving. When first used it transformed an unknown induvidual into The Destroyer, and gave them its power while manipulating it to erase Zach from time for unknown reasons.

Prophecy
A mural was drawn from the legend of time. It depicted a draconic beast battling a humanoid in a jet of Boost aura. The Ultimate power is recreated every time Zach re-enters the timeline, and so must be defeated by him to destroy The Ultimate Power. As Maddy and Hopey have both stated "Someone or something knew you would be standing here, and they knew for thousands of years before today."