The Imperial Guard were the personal guard of the Emperor of Kehjistan. They were quartered in Caldeum and served Hakan II, who created them as an alternative to the Iron Wolves. They were outfitted by the Foushee weaponsmiths.
In addition to guarding the Emperor, they also patrolled the city and surrounding regions. The Imperial Guard obeyed only the Emperor, and could use deadly force against any citizen, poor or rich, noble or commoner, at his order.
Prior to the End of Days, Belial (disguised as the emperor) replaced most, if not all of the Guard with his Deceiver demons. Using this disguise, they prevented refugees from entering the city of Caldeum, and were guarding the Khasim Outpost, slowly pushing the Iron Wolves who tried to keep the situation under control. Imperial Guard were also robbing the people of Dahlgur Oasis for supplies.
At one point, the Imperial Guard was successful at tracking down and capturing Leah in the Sewers of Caldeum, bringing her to the Emperor's trial. Though the Nephalem rescued her after a brief fight, Emperor Hakan later admitted that the Guardsmen were to kill him should he had sided with the intruders.
The player encounters Imperial Guard multiple times during Act II of Diablo III, most notably in the Imperial Palace and City of Caldeum zones. When they notice the player, they assume their real form in a short time, although before that they have a brief moment of fighting in their current form. Some of the guardsmen are dressed as officers, but as they are all demons in disguise, it gives them no advantage.