The Kingdom of Quel'thalas has been home to the High Elves of Silvermoon city for ages. Home to the Sunwell, Arthas marched his Scourge forces upon the area and assaulted the defenses led by Sylvannas Windrunner. The elves' three-tiered "elfgates" defense failed, and Arthas destroyed the sunwell to resurrect Kel'thuzad.
The elves, left in disarray following Arthas' sudden departure, gradually reformed their kingdom under Prince Kael'thas Sunstrider. After a series of betrayals by the Alliance, the elves seceded and renamed themselves Blood Elves in memory of their fallen brethren. Kael'thas has traveled to Outland and left the regent Lor'themar Theron in charge in his stead.
Quel'thalas itself consists of Eversong Woods and the Ghostlands, and, arguably, portions of Eastern Plaguelands. These regions were once all lush, fertile forests, but only Eversong Woods remains this way. Ghostlands has become an eerie, distorted marsh, and the Plaguelands are blighted beyond belief. With the aid of their new Horde allies, the Blood Elves are finally able to recapture control over most of their kingdom, but they still need to contend with the Amani trolls in the southeastern portions of Ghostlands, as well as Scourge remnants and some Alliance detachments that have been scouting their forces.