From Destinypedia, the Destiny wiki
To face one of the Headless Ones, the players must complete one of their summoning rituals which will spawn in a random area around the Lost Sector. These rituals have a set timer and if they are not completed, the players will miss an opportunity to kill one of the Headless Ones. Whether or not if the Guardian is near the ritual or on top of it, as the Headless One spawns the players will be flung back from the ritual.
Unlike other Knights, the Headless Ones have two new abilities. These include the Darkness Barrier in which instead of summoning a single Wall of Darkness, they summon in three to add in better coverage from incoming fire. They also summon in Poison Clouds which were an ability only exclusive to Hive Wizards, and can lob Acolyte Grenades.
List of appearances