From the introduction:
In the long years that Aggeron the Great ruled the vast Empire of the Suns he never once gave pause to consider the loyalty of his slaves. In their multitudes they laboured, their lives spent in toil and privation, their obedience assured by the Word of Command that enslaved them to his will. For his fellow Trell'sara however, there was no such comfortable loyalty. Deceit and treachery were the coin of his dominion, murder and betrayal the tools that assured advancement within the great Empire that Aggeron had created for himself. To counter the ruthless ambition of his fellow Masters, Aggeron gathered to himself a Hammer Guard, a unit of highly trained soldiers with which he rooted out his enemies and destroyed them. Feared by all who might oppose him they came, in time, to be known as Aggeron's Hammer.