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This article contains plot details from the latest books.
Reason for Wearing Edit
Dex created one for himself later because he blamed himself for hurting Sophie with his gadget after she warned him about it. Sophie made Sandor crush it into a ball and told Dex to never make one ever again.
Dex made it not knowing that is was for Sophie, and the councilors ordered him to. Dex was trying to make a device that could give Talentless elves powers, so other's wouldn't have to face the scorn his parents faced, but the Councillors requested him to change it into an Ability Restrictor.
Dex also said that he just wanted his family to be respected so that his younger siblings wouldn't have to go through all the scorn he went through. He said that this assignment gave him a chance to be respected.
The ability restrictor only hindered Sophie's abilities and did not completely shut them down.