Prince Kael’thas is the last royal line of Quel’thalas (Northeast of Lordaeron is the elven nation) and member of the Kirin Tor ( Faction of mages ruling over the city of Dalaran). The prince was studying the magics of the Dalaran at time of the invasion of the Scourge. When the prince returned to Quel’Thalas, the region was destroyed. He noticed that the elves were not feeling well when the Sunwell’s magic was drained. Kael’thas gathered up the elven survivors and created the sin’dorei in honor of the elves who’ve fallen to the the Scourge. Kael’thas led a campaign to eliminate the Scourge in Lordaeron. Kael’thas also helped the Nagas free Illidan Stormrage. Illidan taught the Blood elves including Kael’thas alternative magic sources. Kael’thas then became corrupted with the reliance on fel energy which was dark demonic energy from the Burning Legion.
Video version of the Story of Prince Kael’thas: