To activate the final boss, you need to complete the mini event before:

1. Defeat the boss in front of the cages.
2. Open the cages.
3. Slay all the mobs that spawn in front of the stairs.
4. Talk to the goblin.
5. Clear all mobs between the goblin and the final boss' path.
6. When the goblin opens the door, the final boss becomes active.

Between steps 5 and 6, you can skip the door as a DH with a couple of simple jumps to get to the final boss faster. Items used:

• Wearing: Onyx Amulet with Wind Sculpting Stone, 3x Azerite Armor with the Longstrider trait, speed enchants
• Before entering: Bear Tartare, Charged Phial of Alacrity
• During run: Oddly-Shaped Horn, Nitro Boosts

Note that the Oddly-Shaped Horn (the large temporary mount) can only be used out of combat. This is where the perk of being a Night Elf comes in: You can just use Shadowmeld to get out of combat.