From Destinypedia, the Destiny wiki
Loaded Captains are a Major variant of Fallen Captains that serve as High-Value Targets in the Gambit  and, formerly, Gambit Prime activities. They are distinguished from other Captains by the white, glowing aura that surrounds them.
Loaded Captains will appear when you have been pit against the Fallen in Gambit, meaning they can only spawn on the Emerald Coast and Deep Six maps. They will only appear once per match, so you and your Fireteam will have one shot at eliminating them before they are gone.
Not long into the match, the Drifter will announce that a High-Value Target is on the field. Once they have arrived, they will begin to quickly make their way to the opposite side of the map. Players will have a limited time to the kill them before they reach their destination. If this happens, the Drifter will say that they target is preparing to depart, and you will only have a few seconds left to kill them before they despawn. Although the target has a large amount of health, they will bleed Motes of Dark as they take damage, and if you are successful in killing them, they will drop a substantial amount of Motes.
List of appearances