“The expansion’s name refers to the return of the “Burning Legion”; a vast army of demons being one of the main, recurring antagonist forces in the Warcraft-universe and whose last invasion was defeated in Warcraft III: Reign of Chaos. This legion and its allies are the main enemy which players will fight against in The Burning Crusade. In addition to some new areas on Azeroth, this expansion mainly features the ravaged world of Outland which the Burning Legion and other powerful beings control.

The Doom Lord Kazzak reopened the Dark Portal to Outland, flooding Azeroth with the ravenous demons of the Burning Legion. Expeditions from the Horde and Alliance, reinforced by their new blood elf and draenei allies, passed through the gateway to stop the invasion at its source. On Outland’s desiccated Hellfire Peninsula, the Alliance discovered several of their heroes who had crossed through the portal many years before, while the Horde made contact with the Mag’har (uncorrupted) orcs who had not participated in their race’s original invasion of Azeroth. The expedition into Outland dragged Horde and Alliance armies further into conflict with the agents of the Legion and the lieutenants of Illidan Stormrage, who had claimed the shattered realm for his own.”

- Source: Wikipedia

Further credits:

Wrote and voice directed Patch 2.1 The Black Temple trailer

Wrote Patch 2.3 The Gods of Zul’Aman trailer