Clitus was an officer in the Companion Cavalry and became commander of the king's bodyguard.Cl;itus saved the life of Alexander during the Battle of the Granicus (334). When the commander of the Companion Cavalry, Philotas, was executed for treason, Clitus was the logical replacement, but Alexander chose to split the command between Clitus and Hephaestion.
At a drunken party, Clitus said something to offend Alexander the Great. Alexander took a lance and murdered Clitus. From then on, Alexander's followers had to be very wary of what they said.
Source: Clitus

