A mommet, officially known as a simulacrum, is a doll or figurine created using Sympathy. If created properly, the mommet can create a link between the person and the mommet, allowing the creator the opportunity to manipulate a person without touching them. A mommet can be made from wax or clay, and can be imbued with a person's blood or hair.
The creation of a mommet is considered Malfeasance, due to its potential to harm others. Despite this, they are relatively simple to make, with many students of The University being able to create them, including Kvothe, Fela and Ambrose.
During the course of the story, Devi, a gaelet who operates out of Imre, takes samples of blood from students of The University as collateral for her loans. Because of this, Kvothe wrongly accuses her of creating a mommet of him during The Wise Man's Fear after a series of attacks are made against him using one.  The perpetrator is eventually revealed to have been Ambrose Jakis, who created a clay mommet using blood from Kvothe after he broke into his room to steal back Denna's ring.