The Demon Within

The Demon Within manifests when the spirit of Sargeras tries to take control of Illidan Stormrage's body during the Gul'dan encounter in the Nighthold.

• Additional adds that spawn are Soul Fragment of Azzinoth, Demonic Essence, Purified Essence. The Purified Essences can cast Purified Essence, becomes Purified at upon reaching full health, instantly killing all players and applying Demonbane to all enemies.

• The Nighthold: Gul'dan

• Like Illidan himself, the Demon Within is voiced by Liam O'Brien.
• It uses a different demonhunter model than the original or the updated one.

It's possible that this is Azzinoth using Illidan's form. This is supported by the Soul Fragments of Azzinoth that spawn using the fight.