The Ability Restrictor, created by Dex, is a silver circlet with swirling design that Sophie was forced to wear in Everblaze, for her punishment for reading King Dimitar's mind. It has a peach colored stone set into it.
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 the ability enhancer, but when the Council learned, Terik was sent to order him to create an ability restrictor. Unbeknownst to him, the Council had ordered it to be made for Sophie, after she had paid a visit to her San Diego home alongside Keefe. However, when Sophie was forced to try it on, Dex learned the truth and made a last minute failed attempt to refuse to help.
Dex said that he just wanted his family to be respected so that his younger triplets wouldn't have to go through all the scorn he went through. He said that this assignment gave him and the Dizznee name a chance. In addition to this, the Council later threatened his family, saying that if he did not put it on Sophie, they'd all be exiled.
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. He showed it to Sophie during Foxfire, however she didn't take the invention well, and she and Dex started to quarrel. Dex also made this enhancer because he was jealous of Fitz and his relationship with Sophie, wanting to outdo him.
The ability restrictor only hindered Sophie's abilities and did not completely shut them down. It also gave Sophie very painful headaches. She briefly falls unconscious when it is first put on before Dex modifies it to hurt her less.