The Ancient Darkspawn is a very old hurlock who appears during An Admirable Topsider. It has donned incredibly durable armor and has hit points rivalling the equivalent level of a drake. It can easily destroy an entire party. So far, they have only been seen in the Dead Trenches.
You can find the hilt and pommel before the quest is fully active. However, the Ancient Darkspawn will not appear until you have both other pieces and visited the warrior's grave in Ortan Thaig.The spell combination Storm of the Century is very effective at reducing the health of the Ancient Darkspawn, especially if another spell can ensure it remains within the area of effect (e.g. earthquake).The party can return to the Legion of the Dead soldiers and allow them to kill it. See: Dragon Age: Origins