A drill thrall was a descriptor given to an individual on Triskelion who was responsible for training thralls. They answered directly to the master thrall.
When the landing party from the USS Enterprise was diverted prior to transport to Gamma II, instead to the planet Triskelion in 2268, they were given their own individual drill thralls, personally selected for each: James T. Kirk had Shahna, Pavel Chekov had Tamoon, and Nyota Uhura had Lars.
A drill thrall was responsible for attending to each new recruit, such as by providing each thrall with nourishment, and assuring they were well trained for combat. Once selected, a drill thrall could not be refused or reassigned. Kirk liken a the term to a top sergeant. (TOS: "The Gamesters of Triskelion")