In Erec et Enide, Bors had been sent on a mission to rescue Guenevere’s cousin from Nabon’s castle, but he himself ended up being captured by the enchanter. Nabon keeps him imprisoned by magic.
Erec eventually comes to Bors’ rescue and defeats Nabon, freeing Bors and other prisoners.
Source
Erec | Chrétien de Troyes, late 12th century