The ArchDoomKnight (ADK) is used for a variety of questlines, it's pretty much required for progression. Use the buy back if you farmed this already.

You are required to have Rank 7 Evil Reputation, please check Faction Reputation Guides if you are not Rank 7.

• "Gathering power"
  • 1800x Undead Energy, dropped from most undead.
    • You need to complete upto the "A Loyal Follower" quest for Blinding Light of Destiny quests in order to start collecting Undead Energy.
    • Fastest to obtain from /necrodungeon 5 headed dracolich.
  • 500x Human Souls from /noxustower.
  • 600x Dragon Energy, drops from most dragon kin, I recommend killing mobs at /etherwardes.

Really unnecessarily long.

• "Death's Door"
  • Grab death's power from /deathsrealm
  • Kill death from /shadowattack x400

Basically kill each chaos lord for a drop

• "Chaotic Lords"
  • Chaotic Power x13 should drop from each of the chaos lords while you're farming them for items.
  • Kill escherion at /escherion for his helm.
  • Kill vath at /stalagbite for his sword.
  • Kill kitsune at /kitsune for the dragon koi blade.
  • Get wolfwing's armour at /wolfwing.
  • Get the One Eyed Doll Breaker from kiberly at /palooza.
  • Head to /ledgermayne and take his armour.
  • Go to /djinn and take the armour
  • Get the Soul of Chaos armour from /dreamnexus
  • Grab the Chaos Lionfang armour from /stormtemple. This is a low drop rate.
  • Get the storm chaos king crown from /ultraalteon. Easiest here since there's usually a group, but go to regular if you struggle.
  • Grab the Xiang Chaos armour from /mirrorportal.
  • Grab drakath's sword from /ultradrakath. It is a 1% drop so good luck.
  • Get the Chaorrupted Hourglass from iadoa in /timespace.

• "A Means to an End"
  • Get the ultimate darkness gem from /shadowfallwar.
  • Grab x2000 Undead Energy from the 5 headed dracolich at /necrodungeon.
  • The Necro Scroll of Dark Arts is a really low drop from the /epicvordred map. It also drops from the originul void quest.
  • Doom heart drops from Ultra Sepulchure at /sepulchurebattle.
  • The Dread Knight Cleaver is dropped by Dark Sepulchure at /sepulchure.
  • Defeat the reaper at /thevoid for his soul.
  • Kill desolich for his eyes at /desolich. Get a group for this.

You should now have the entire Archdoomknight Set.