Sapphiron

Sapphiron was an ancient agent of the blue dragonflight who was killed by Arthas Menethil and raised as a powerful frost wyrm. He was subsequently sent to the halls of Naxxramas, where he guarded the inner sanctum of the citadel's dread master, Kel'Thuzad. Years later, Kel'Thuzad summoned him once more in the afterlife of Maldraxxus.

Biography
Sapphiron was one of the most ancient members of the blue dragonflight, and one of those entrusted with a key to the Focusing Iris by Malygos himself. Like many of the blue dragonflight, Sapphiron had immense magical power. The wyrm lived with his brood of blue dragons near the entrance to Azjol-Nerub in the mountains of Northrend, where he guarded a hoard of powerful artifacts he had collected over the centuries.

When Arthas Menethil and Anub'arak were preparing to journey through Azjol-Nerub to reach Icecrown and aid their master, the Lich King, Anub'arak suggested that Sapphiron might hold artifacts that would protect them on their march. The dragon confronted the Scourge forces when they entered his lair, and found Arthas' honesty about their intention to kill him and steal his treasure to be "refreshing". The undead managed to kill the wyrm and his dragons, after which Arthas used some of his dwindling necromantic power to raise Sapphiron as a frost wyrm to serve the Scourge and help them destroy the forces blocking the passage to Azjol-Nerub. Sapphiron was called upon again to destroy the dwarves guarding the gates to the Spider Kingdom, but was left behind outside when the Scourge ventured inside, since, as Arthas put it, "the confines of the dark earth" were no place for him.

Years later, Sapphiron was stationed in the dread necropolis Naxxramas, where he protected the Frostwyrm Lair against intruders seeking to reach the chamber of the citadel's master Kel'Thuzad. With his magical powers having been amplified when he was resurrected by Arthas, Sapphiron used his frost magic to defend Kel'Thuzad at all costs. Even in undeath, he continued to hold onto his key to the Focusing Iris, which was retrieved by adventurers when they defeated the frost wyrm on their way to Kel'Thuzad.

Another several years later in the afterlife of Maldraxxus, Kel'Thuzad and Baroness Ninadar betrayed Margrave Sin'dane to usurp control of the House of Rituals, an event which was witnessed by Balmedar and the Necrolord Maw Walkers. Before departing to Exoramas, Kel'Thuzad summoned Sapphiron in the form of a fleshy undead dragon to deal with Balmedar and the heroes, but they managed to defeat the dragon.

Abilities

 * Naxxramas


 * During The Traitor Unmasked

Objective of

 * The Traitor Unmasked

Encounter with Arthas

 * Sapphiron: You dare enter my lair? I am Sapphiron, ancient servant of Malygos the Spell-Weaver! Explain yourselves!
 * Arthas: Sorry we don't have time to chat, great wyrm. We've come to murder you and steal whatever artifacts you've been hoarding over the centuries.
 * Sapphiron: Honesty... how refreshing. None may challenge Sapphiron and live!

Speculation
Despite not being a willing agent of the Scourge and thus ally of Kel'Thuzad, Sapphiron doesn't hesitate to respond to his summons in Maldraxxus. While not directly stated, it is possible that Kel'Thuzad simply forcibly took control of Sapphiron at some point after they were both within the Shadowlands. Alternatively, the dragon may have developed a form of Durnholde Syndrome during his time as a frost wyrm.