The Rending

The rending is the name given to the ritual The 8 conduct to disconnect a firstborn from their familiar and therefore their magic. After the Thinning Xn’thana ordered that the non-firstborn go through the ritual at age 12.  
Very few have witnessed the ritual outside of The Chosen During the ritual, the child (or magic-user found to be outside the law of magical use) is restrained and the Illithid places a creature that resembles a slug whose toothy maw encompasses the bulk of its girth at the head on the victim's eye. The slug then slithers under the eye. At this point the victim and their familiar scream in agony. Then the familiar dies and the victim is left without their magic. 
No one knows why this happens. Usually, if a Chosen or its familiar dies the other does as well. People theorize that this is because their life forces are connected via the magic bond. Somehow this must be broken during The Rending. Not many who have lost their magic are willing to talk about the experience. Some believe that the victims actually die due to the sometimes morose and downtrodden deminer of those who have gone through the ritual and lost their powers.