The Dominion's engineered soldiers who depend on ketracel-white.
The Jem'Hadar are the elite soldiers of the Dominion, a species bred for combat and for absolute loyalty. They are strong, fast and tough, and they grow to adulthood within days of being born. They need a drug known as ketracel-white to stay alive, and the Vorta who supply it use that dependence to ensure obedience.
The Jem'Hadar believe that their Founders are gods, and that it is their duty to serve and die for them. Their discipline and fearlessness make them very dangerous, and they have no concern for their own survival in battle. Their society is wholly military in character.
The Jem'Hadar first appear in Deep Space Nine, and play a major part in the Dominion war against the Federation, the Klingons, the Romulans and the Cardassians. The episode The Abandoned introduces a young Jem'Hadar whom the crew attempts to raise. Vorta such as Weyoun usually supervise them in the field.