Night CourtG 20

For completing the "Court Of Oryx" questline in the Taken King Expansion. You can obtain this quest by participating in one run of Court of Oryx then heading to Eris in the Tower, who will then give you the quest. Note that there will not be a marker over her head telling you that she has a new quest for you. Completing this requires defeating 6x Tier 1, 3x Tier 2 and 1x Tier 1 bosses at the Court of Oryx on the Dreadnaught in Patrol mode. Tier 1 require a Reciprocal Rune to start (can be found in chests or drops on Dreadnaught), and are not too difficult. Tier 2 require an Stolen Rune, which is a drop during Tier 1 events. It must also be "charged" by doing 3x Tier 1 events before you can use it. These are recommended light level 240. Lastly, TIer 3 requires an Antiquated Rune, which must be charged by doing Tier 2 events, similar to the Ascendant Rune. These summon level 42 enemies, and you'd best have a group of light level 290's minimum to get these done.

What makes this much easier to complete however, is the fact you can "piggy-back" on others, meaning you can simply head to the area and if others are already running it, you can join in and get credit. I got all of my Tier1, 2x Tier 2 and about 8x TIer 3 done by doing this. You also still get rewards, just not as nice as if you were the one using the summoning rune to start the fight. The end result of all of this, is not only the achievement, but a level 300 class Artifact, which you can pick up from Eris when done. These also gain Eris experience to level up with her.