Biology
The Dethaki were tall and thin with feathered bodies, sharp beaks and clawed hands and feet. Despite their thin frames, they were very tough and could survive a large amount of damage. Though they had bird-like creatures for ancestors, they had long since lost the power of flight. They had screeching voices. (BNA: Mean Streets)
Culture
The Dethaki had a strong sense of honour. They would always give the people they were sent to kill a chance to fight back and would supply a sword for this purpose. They would also only return from a mission successfully, having either killed the target or died in the process. If they were greatly dishonoured (generally by failing in the kill), the Dethaki would ritually disembowel themselves. (BNA: Mean Streets)
Dethak used laserblades in their killing. (BNA: The Medusa Effect)
History
The Dethaki had to move into the mountains of their home-planet when mining corporations took over their land. They were unable to fight back against them and were forced to take jobs as killers. They made excellent mercenaries and assassins, but they were too temperamental for long-term military positions. (BNA: Mean Streets)