From Destinypedia, the Destiny wiki
The Treasured Knights are Ultra Knights seen in the Hunted encounter and during the Arunak boss. There are two variants of the Knight: the Darkblade variant seen in both encounters and the normal Knight seen only in the Hunted.
The Darkblade variant often appears within the Hunted encounter, deployed to clear anyone off the Lamp plates. They have a large health pool and deals a devastating Area-of-Effect attack. The normal Knight appears only when the middle plate is being controlled, laying into the team with it's Boomer while slowly approaching the plate for a Fist of Darkness. Killing these Knights provide points to the progression bar, so killing them is essential. The Darkblades also appear during the fight with Arunak, Beloved by Calus once it's health reaches a third. They can be killed, but not doing so can provide a triumph once Arunak is killed.
List of appearances