Smoldering Ember Wyrm

Enter Return to Karazhan through the gate on the tower set to Mythic difficulty — not Mythic+ or Heroic, as those split the dungeon into upper and lower halves and do not contain all five crystals needed to summon Nightbane.

You have 8 minutes to reach the first crystal, so head immediately to the Opera Hall and defeat the Opera boss, which is a randomly selected encounter.

Exit the Opera Hall and move to the audience hall, clear the mob there, and collect the first crystal.

Run to the last room on the left before Maiden of Virtue — skipping Maiden entirely — clear the mob, and collect the second crystal.

Jump down from the lodge into the Moroes room, kill Moroes, and collect the third crystal; if it is your first run, also pick up the keys Moroes drops on the floor.

Exit the Moroes room, run down the stairs to the front yard, turn left into the spider room, go all the way to the room in the top right, clear the spiders, and collect the fourth crystal.

Locate and collect the fifth and final crystal to complete the summoning requirements for Nightbane.

With all five crystals collected, Nightbane will be summoned — defeat him to have a chance at the Smoldering Ember Wyrm, which drops once per party at effectively 100% when all five eligible players in the group do not yet own the mount and are not loot-locked.