Board Thread:Lore Discussion/@comment-10209824-20130530153039/@comment-16047389-20130917211529

Maybe after Artorias broke his arm, left his shield behind, and exhausted his strength fighting the creatures of the Abyss, Manus was finally able to fight him on fair grounds. But even then, Manus couldn't win, so he cheated and corrupted Artorias in his weakened state. So when the Chosen Undead finally faces Manus, he's been tired out by Artorias and doesn't stand a chance.

Also, maybe the Abyss just doesn't affect Undead they way it does everything else.