Main prerequisite before all this is that you have Rank 10 Doomknight in your inventory in order to do the ”The Doom the Looms” quest. You also need to be member to even start this questline, but the armour itself is non-mem.

You can start the questline at /necropolis from Vayle.

• ”Sepulchure’s Armour”
  • Grab the DoomKnight Hood from the doomknight shop in /shadowfall
  • 40x Dark Spirit Orb
    • Obtained from doing the Dark Spirit Orbs quest