From Destinypedia, the Destiny wiki
Lost Knights can be found in the Ley Line, Vesta area of Astral Alignment. Once the nearby Beacon has been located and activated, Lost Knights will spawn in with other Taken reinforcements to defend the area. Players will be unable to damage Lost Knights as they are protected by an immunity shield which requires a total of 3 paracasual batteries to be deposited onto 3 plates.
Players are tasked to replace the batteries that charge the Blind Well's energy regulators. These batteries are also unique for its paracasual properties and collecting one will give them an "Encumbered" buff that slows down movement speed and lasts for a duration of 20 seconds. Due to the weight of the batteries, players will experience an "Exhausted" debuff that prevents them from carrying the battery until the exhaust has worn off after 10 seconds. Players are required to deposit these batteries onto 3 plates to deactivate the immunity shields of the Lost Knights, and to charge the Blind Well.
Lost Knights are armed with a Stasis Boomer, which is capable of blasting miniature Duskfield grenades. This has the ability to slow the movement speed of players and if they stay too long inside the Duskfield grenade, the slow multiplier will increase to 100 and freeze them for a duration of 2 seconds. The Lost Knight will use this opportunity to blast Solar projectiles that will apply a damaging Burn.
They attack from a mid-ranged distance and will regularly teleport to be within its attacking range. Getting too close to the Knight will prompt it to unleash a devastating ground smash attack which can hurtle players off the cliffs or even cause death. The Lost Knights are highly durable and require a significant amount of damage to defeat.