Source : Webster's Revised Unabridged Dictionary (1913)

Taskmaster \Task"mas`ter\, n.
   One who imposes a task, or burdens another with labor; one
   whose duty is to assign tasks; an overseer. --Ex. i. 11.

         All is, if I have grace to use it so, As ever in my
         great Taskmaster's eye.                  --Milton.

Source : WordNet®

taskmaster
     n : someone who imposes hard or continuous work