Kael'thas Sunstrider

Kael'thas Sunstrider is the former prince of the High Elves and current leader of the Blood Elves.

Considered a traitor for allying himself with the Naga and with the also considered traitor Illidan Stormrage, he is currently in Outland allied with the Illidari in the hope of finding some way to save his people from the magical addiction that afflicts them, after the fall of Quel'thalas and the destruction of the Sunwell.

Background
Kael'thas spent most of his time in Dalaran, as he was part of the Council of Six, a council that very few knew existed, and as a High Elf and the relationships that humans and Elves had formed since the Troll Wars had earned such a privilege to be part of the government of a city shared between humans and High Elves.

Kael'thas was in Dalaran when the plague swept through Quel'thalas, killing most of the High Elves, corrupting the Sunwell, and killing King Anasterian Sunstrider.

Upon hearing of the attack, Kael'thas hurried back to Quel'thalas only to find such a sight.

He immediately gathered the surviving High Elves and, in honor of those who had died, renamed them Blood Elves.

With a regiment of Blood Elves Kael left for Lordaeron to help the remnant of the Alliance that was still fighting against the Scourge, but Marshal Garithos, with a strong hatred towards the elves, sent them on suicide missions, after allying with Lady Vashj, Garithos sentenced Kael'thas to death after learning of such an act of betrayal, but Kael'thas managed to escape from the catacombs of Dalaran before being executed.

After this, Kael allied himself with Illidan Stormrage, in his attempt to conquer the Frozen Throne, but upon being defeated, Kael took Illidan and along with Lady Vashj and several Blood Elves set out for Outland, where they remain to this day, considered a traitor by his people and left for dead.

Mysteries of Azeroth
Kael'thas remains in Outland, allied with the Illidari, seeking a way to cure the addiction of those he considers his people, both the High Elves and the Blood Elves.

Many consider him dead, since they saw how the promise of his return faded over time, and with the worrying news of his Alliances with the Naga and with the Demon Hunters, he is maintained as a traitor among the High Elves of Alah'thalas.

Kael keeps an eye on the Elves of Alah'thalas through the Felstrider Retreat, a Blood Elf settlement in Alah'thalas, and by maintaining direct communication with a member of the new Regency Council, Grand Magister Rommath, who still maintains the faith in his prince despite everything.