You play as a darkspawn vanguard (the leader types with the horned helmets) during the last battle in Denerim. He's a sword and shield tank with some abilities from the champion spec. You lead a group of darkspawn through the city, fighting every companion from Origins along the way, one by one. Oghren will come storming out of the inn with some drunkards, Wynne guards a gate with some templars, etc. Nothing terribly interesting. The ability to recruit darkspawn on the fly and gain their approval by getting your hands dirty is mildly amusing. If one dies you just grab another one. They constantly spawn near you and follow your main group around, so you always have a steady supply of troops. You'll be using Ogres to smash through barriers and Shrieks to sneak by sets of ballistas. Nothing terribly exciting.
So you fight your way through the city until you reach the palace, fight a group consisting of Alistair, Morrigan, Leliana and a bunch of Werewolves to protect the Archdemon. You slaughter them all and that's it. The end. You see your guy finishing off Alistair and that's pretty much all that happens if 'your PC from Origins hadn't survived the joining, yadda yadda'. Bioware making things sound more compelling than they end up being, case #X.
Lest we forget you also get a sword for Origins and Awakenings. Useless to anyone but those who are actually still playing the campaign of either.
tl;dr: It's rubbish.
This mod might actually be more entertaining. Made by a GAFer, too. :lol