The Demon Within

The Demon Within manifested when the spirit of Sargeras tried to take control Illidan Stormrage's body in the Nighthold.

Abilities

 * 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.
 * 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.
 * 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.
 * 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.
 * 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.
 * 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.
 * 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.
 * 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.
 * 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.

Objective of

 * The Nighthold: Gul'dan

Trivia

 * The Demon Within is voiced by Liam O'Brien.
 * It uses a different demonhunter model than the original or the updated one.

Speculation
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.