sentences:  During his service time, he was a member of the 'Hell-Hounds', a military unit that is said to be the closest version to a mercenary group that the U.S. has ever fielded in battle.Mercenaries or "Mercs" are a recurring characters in "The Chronicles of Riddick" Universe.In this setting, Mercs are often found filling the roles of Bounty Hunters who take out contracts to capture the universe's most dangerous criminals and turn them over to brutal Triple Max Prisons.