Kathra'natir

Kathra'natir, also spelled Kathra'Natir, is a powerful nathrezim who has been archenemy of the Council of Tirisfal ever since the creation of the first Guardian. He was eventually tricked into possessing Meryl Felstorm, becoming imprisoned in Meryl's body, but during the Burning Legion's third invasion of Azeroth he broke free to threaten the Council once more, leading to the reforming of the Tirisgarde.

Navane
Kathra'natir was the last dreadlord to wield the ancient nathrezim blade known as Apocalypse. The weapon passed to him during the Burning Legion's invasion of a planet known as Navane, home to a handful of intelligent races. Though they shared a history of enmity, they put aside their differences to form a tenuous alliance against the Legion. Disguised as one of their own soldiers, Kathra'natir walked among Navane's resistance forces and whispered rumors of betrayal in every ear while Apocalypse clouded the defenders' ability to reason. Old hatreds and rivalries resurfaced, and the defenders began to fear each other as much as the Legion. In one night of wanton bloodshed, Navane's army went to war with itself as Kathra'natir admired his handiwork from a distance. By sunrise, not a single soldier remained to oppose the Legion's conquest of the planet.

War of the Ancients
Kathra'natir participated in the War of the Ancients, but was destroyed by Malfurion Stormrage. His history and description was eventually recorded in the Compendium of Terrors, a record accessible to the Council of Tirisfal.

Legends: The First Guardian
2,610 years before the First War, Kathra'natir infiltrated the city of Dalaran. He spread plagues and disease, and used Apocalypse to warp the minds of the non-mage civilians, turning them against the magocratic rulers of the city. He stole the Amulet of Waters from the Council, and through it he was able to control and corrupt Lordamere Lake. He was later banished by Alodi, the first Guardian of Tirisfal and the Council of Tirisfal. When he was banished, his sword Apocalypse was left behind.

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Meryl thought he was gone, banished, too weak to trouble Azeroth again, but he returned when Vendellin Soulfire, hired by Katrana Prestor, cast the Mark of Kathra'natir over Valeera Sanguinar, allowing him to possess her.

During Med'an's rescue at Ahn'Qiraj, he managed to get free and get control of Valeera to fight Cho'Gall. Later, when Meryl teleported them away, he tried to possess Med'an; but Meryl Winterstorm offered himself in his place, taunting him about his abilities. The demon fell into the "trap" and possessed Meryl instead, becoming contained once again.

Since then, the dreadlord has been inside Meryl, forced to do his bidding, and sometimes helping him with his own abilities. Eventually, Meryl thought the influence of the demon too much for his relationship with the New Council of Tirisfal and he left.

Meryl Winterstorm later went to Ahn'Qiraj to help Med'an and the New Council of Tirisfal. During the battle Meryl absorbed the fel energies that had tainted the Base of Atiesh so that Med'an could wield the greatstaff, but the influx of power gave Kathra'natir the strength he needed the break free. His spirit immediately went after the source of Med'an's power: the council members channeling it into him from within the Twisting Nether. He effortlessly slew the blood elf Dalynnia Wrathscar and drained her magic into himself, but was ultimately defeated and chained within Meryl once more. Meryl, knowing that Kathra'natir could break free again, exiled himself rather than place the council at risk again.

Legion
Despite Meryl's best efforts to subdue Kathra'natir, the dreadlord managed to break free. He is one of the many demons summoned by in front of the Tomb of Sargeras to attack the forces of Azeroth in the climactic, disastrous battle at the Broken Shore. Kathra'natir later attacked the Violet Hold in Dalaran, and with the aid of a mage adventurer and the spirit of Alodi, Meryl fought Kathra'natir off, but Kathra'natir escaped into the Twisting Nether before he could be killed. Meryl and Alodi gathered the Forge of the Guardian, and within the Hall of the Guardian Meryl reformed the Tirisgarde--a group of magi used by the Council of Tirisfal in the absence of a Guardian--and declared that its goal was to hunt Kathra'natir down.

He later possesses Vargoth and uses the Nightborne Soulstone along with the power of the Nexus to drain Meryl of his power, which manifests as a sickness that puts Meryl out of commission. When Vargoth is freed, he and the champion of the Tirisgarde trap Kathra'natir inside the Nightborne Soulstone.

While he is powerless inside of it, Kathra'natir's power is siphoned from the Nightborne Soulstone to power a weapon defense system located in the tallest spire atop Dalaran's Violet Citadel.

Abilities

 * Oculus
 * Assault on Violet Hold
 * Assault on Violet Hold
 * Assault on Violet Hold

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 * "I am not her demon. She is my vessel! She offered herself to me willingly! And this is no child's game!"
 * "Meryl Winterstorm! It's been a long time since we met!"
 * "Powerful though you are, mage, you are no guardian! You have not the power to command me!"
 * "I hear it too, boy, but the voice is only an echo of the past! It promises death... while I offer you life... and powers beyond redemption! Surrender to me and become greater than the gods themselves! Come! Embrace me. And be mine!"
 * "You offer me a place inside you... thinking yourself powerful enough to control me? I am a dreadlord!"
 * "If it is a battle of wills you want, mage... we shall see whose power is greater!"
 * ''"She's right you know, you spend your magic recklessly to control me while I waste my own power fighting you. Why squander all our energy thus? Why not a partnership instead? Let me help you fight that monster...for fel magic is powerful...and we are near the sea. It requires but an insignificant sacrifice--a living fish or two, a few strands of seaweed--to fuel the magic. What are these small lives if you save the boy?
 * "The mage has fallen! I am free to take what should be mine! To seize that last glimmering strand of power! To--Gone! The boy is too strong now! I cannot approach him! But there is another source of magic! The torrent that pours from the council! Enough to empower! Enough to restore!"
 * "Mage, you are too drained for another teleport just now. Borrow a small portion of my fel strength."
 * "You think to thwart me in this way, mage?"

Trivia

 * Kathra'natir is voiced by Patrick Seitz.