Aethas Sunreaver

Archmage Aethas Sunreaver is one of the members of the Council of Six — the ruling council of the Kirin Tor, and the leader of the Sunreavers. He is found standing next to Rhonin and Archmage Modera in the Violet Citadel. He is a strong supporter of the Horde, and through his efforts, though traditionally barred from Dalaran, the Horde have been granted sanctuary. Also, the Horde district of Sunreaver's Sanctuary is named after him. This refers to his high position within the Kirin Tor. He is the first blood elf known to be part of the Council of Six as Kael'thas was a high elf at that time.

Following a Horde player delivering him the legendary blade Quel'Delar, Aethas will announce that the blade has been restored.

His image resides in the Dragonblight in the Horde fortress of Agmar's Hammer.

In the Shadow of the Sun
Shortly before the Northrend war, Aethas sends several letters to the Regent Lord of Quel'Thalas, Lor'themar Theron, to arrange an audience. Lor'themar deliberately ignores them at first, though Aethas simply announces his intention to visit Silvermoon City either way.

He is greeted by Lor'themar, Halduron Brightwing, and less enthusiastically -- Grand Magister Rommath. Aethas informs the three of Malygos the Spellweaver's intention to wipe out all mortal magic users, and that as such, The Kirin Tor had become the crazed Aspect's enemies. He asks for his peoples' aid in the matter, though Rommath immediately denies him. Referencing the sin'dorei's slated execution in the dungeons of Dalaran, while the Kirin Tor did nothing to intervene, Rommath simply notes that they can all "rot in hell" for all he cares. He assures Aethas that none of his magisters will aid the Kirin Tor, and that as the Grand Magister, Aethas has no authority on the matter.

Lor'themar stops their bickering, and has Aethas shown to his quarters. Following Lor'themar's return from failed negotiations at Quel'Lithien Lodge, Aethas reveals that several high elven members of the Kirin Tor under Vereesa Windrunner share a similar attitude.

Sylvanas Windrunner later travels to Sunfury Spire to enlist the sin'dorei's aid against the Lich King, reasoning that as members of the Horde they are obligated to commit support. Despite Lor'themar's apprehension to commit troops so soon after the battle on Quel'Danas, Sylvanas is able to twist his arm enough to warrant co-operation, citing that their refusal to battle the Lich King would likely lead to further death in Quel'Thalas -- and the loss of Horde support.

With the sin'dorei's presence in Northrend confirmed either way, Lor'themar agrees to support Aethas' endeavor, though makes a point that it will not be on behalf of the Kirin Tor. Lor'themar instructs Aethas to confer with Rommath, a notion Aethas expresses will probably not end too well.

Speculation
Unlike his Alliance equivalent Vereesa Windrunner who is Silver Covenant faction, his in-game faction is Kirin Tor and not Sunreavers. This is very likely because he is member of the Council of Six, effectively making him one of the rulers of Dalaran, whereas Vereesa is only Rhonin's wife and thus not affiliated with Kirin Tor otherwise.

Trivia

 * At some point, Aethas' seemingly "out-of-no-where" appearance in the position Kael'thas Sunstrider once held was speculated to be more than coincidence- some players suggested that Archmage Sunreaver could, in fact, have been a redeemed Kael'thas, his lack of a head being obscured by his helmet. Though clearly unlikely, a number of lore fans have taken amusement in the thought.
 * Aethas' Intensity is named after him.

Quotes
yell