Sentinels were a type of droid model used by the Galactic Empire and programmed to serve the will of Galactic Emperor Sheev Palpatine. The droids were manufactured in the Imperial base on the planet Vetine, and were capable of playing pre-recorded messages from their head. After the death of Palpatine during the Battle of Endor, multiple sentinel droids known as the Messengers carried the late Emperor’s posthumous messages to Imperial commanders with instructions to initiate Operation: Cinder.
The Emperor’s sentinels have been shrouded in mystery since their introduction in the comic Shattered Empire. The final book in the Alphabet Squadron Trilogy, Victory’s Price, has revealed their true nature, and what makes them tick.