At some point, one of the primordial serpents convinced the inhabitants of a land called Oolacile to upturn the grave of Manus, the primeval man and Father of the Abyss. Upon his awakening, Manus started to expand the abyss, and the inhabitants of Oolacile mutated into crazed demonic creatures. Artorias, one of Gwyn's knights, was sent to stop the spread of the Abyss, but failed, and became corrupted by the dark. Meanwhile, Manus has captured Princess Dusk, and searches desperately for his long-lost pendant across space and time.
In the DLC, Manus grabs the Chosen Undead and pulls them into the past, if they are able to find Manus' broken pendant. Inside the past, the protagonist defeats the corrupted Artorias, destroys Manus, halts the spread of the abyss and saves Princess Dusk. The events become legends, but the protagonist's exploits are falsely attributed to Knight Artorias.
Source: Wikipedia, "Dark Souls", available under the CC-BY-SA License.