Dr. Finitevus's hex is an incredibly powerful magical spell created by Finitevus in his efforts to destroy all life on Mobius as part of Project Enerjak: Reborn. Finitevus originally created the hex during a long period of research and casting, going to great pains to ensure that it fulfilled its purpose: to transform a willing applicant into Enerjak. The hex was placed on the Master Emerald, binding it to Finitevus' will. It also served to block a Guardian's link to, and abilities to harness, the Emerald, negating their natural chaos powers. Finitevus was the only person who could access its power, and did so using a slightly altered chant; "The Servants are the Seven Chaos, Chaos is Power, Enriched by My Will".
The Return of Enerjak
Finitevus's hex was intended for Knuckles the Echidna, who was convinced to tap into the Master Emerald's power to bring down Dr. Eggman after learning of his slaughter of the Echidna population of Angel Island. The hex scrambled Knuckles's mind, and invoked a brainwashing spell, leading him to become a willing participant in Finitevus's plan and being reborn as Enerjak. (StH: #181)
Sonic the Hedgehog, planning to save his friend, attempted to neutralize the hex the same way as he defeated Perfect Chaos: by draining power from the Emerald to become Super Sonic. According to Finitevus and Merlin Prower, it is surprising that Sonic could use the Emerald's power without the hex having any effect on him, as he should not have been able to draw power from the gem. It remains a mystery how he was able to turn into Super Sonic, if the gem's power was "locked" to only Finitevus (the reason why could only be found by dissecting him). Nevertheless, his efforts were futile, as the hex was too strong and resisted his positive aura (while it did have an effect, it wasn't enough). (StH: #183, #184, #187)
In the end, it was discovered that Finitevus, expecting a plucky resistance to "save" their beloved Knuckles, had "locked" the hex, leaving him unable to stop the fight between Super Sonic and Enerjak even if he wanted to; the only way to break the Hex was for someone to sacrifice their own life. Finitevus' belief that no one would be foolish enough to sacrifice their own life for another was soon proven wrong, as Julie-Su, Archimedes and Locke the Echidna all volunteered to do so. In the end, while Archimedes transported Finitevus away, Locke elected to perform the ceremony, wanting to make amends for all he had put his son through. Locke's sacrifice of life-force broke the hex and caused him to be consumed in energy from the Emerald, but Knuckles did return to normal while Finitevus fled to fight another day. (StH: #184)
To be sure there would no ill effects on them because of the hex, Sonic and Knuckles were included in a ceremony by Merlin to remove the curses placed upon Miles "Tails" Prower, Mina Mongoose and Mighty the Armadillo. (StH: #187)