The origin of the Worgen is shrouded in mystery, as it had been stated that they are from a different world and were brought into Azeroth. Velinde Starsong brought them through the Scythe of Elune, a magical weapon she found, thought to be a sign and blessing to vanquish her people's enemies. Archmage Arugal brought them in through dark magic to serve a similar purpose of protecting his people of Gilneas against the scourge during the Third War. They proved too difficult to tame, possibly having killed Velinde, and Arugal went mad from the idea of him helping destroy his own people. Arugal adopted many of the Worgen creatures he helped create as his own, retreating to Shadowfang Keep as Wild Worgen roamed Silverpine Forest - hunting the people of Gilneas City, the walled off kingdom ruled by Genn Greymayne.
World of Warcraft: Cataclysm
The playable Worgen in World of Warcraft all hail from Gilneas, a city state walled off from the rest of Lordaeron in the years of the Third War. Not wishing to take part in the war against the Orcs, and later the Scourge, King Genn Greymane surrounded the city and its outlying lands with a massive barred wall, the Greymane Wall. The wall kept Gilneas safe for many years, until eventually rampant Worgen broke into the city and infected much of the populace with the Worgen Curse. Years later, many of these Worgen were granted sentience over their beast form through the efforts of the Royal Chemist Krennan Aranas, in order to aid the remaining Gilneans in the defense of the city against an oncoming attack by Sylvanas Windrunner and her Forsaken armies.
During this siege, Deathwing, the dragon Aspect of Death, escaped from the core of Azeroth and brought about the Sundering of the world, cracking open the Greymane wall and severely damaging Gilneas and its outlying towns as well. However, the breaking of the Wall allows the Night Elves of Kalimdor to sail a fleet to Gilneas' aid, fighting back the Horde forces and escorting Greymane and the remaining evacuees to Darnassus. It is here that, feeling responsible for the Worgen curse due to Velinde Starsong's role in their creation, the Night Elves induct Gilneas and the Worgen into the Alliance.
Introduced in the Cataclysm expansion pack, the Worgen can play as 8 classes - Rogue, Warrior, Warlock, Priest, Druid, Mage, Hunter and Death Knight. All Worgen characters have two forms, beast and human, which they can transform between outside of combat with a racial ability, Two Forms, however in combat the player character automatically reverts to their Worgen form.