Archdemon

Archdemons are the Old Gods manifested in the form of powerful and terrifying dragons, and tainted by darkspawn. They possess an intelligence far beyond the average dragon and are purely evil creatures.

Normally, the darkspawn are disorganized and fight with each other, rarely appearing on the surface except in raids and small invasions. However, a true Blight begins when the darkspawn find a leader who unites them into a great horde and unleashes them onto the surface in a wave of darkness and destruction. This leader is an Archdemon and a Blight will not end until the Archdemon leading it is destroyed.

Since the dwarves managed to fend off the darkspawn invasion of the Deep Roads after the First Blight, no Archdemon has led a Blight against them.

The first appearance of an archdemon in the game occurs when The Warden drinks the tainted blood of the darkspawn during the Joining. However, when the Warden travels to the Dead Trenches, an Archdemon will be seen in the flesh. The Archdemon in the game was a Tevinter god of beauty named Urthemiel.

The Warden discovers on the eve of the Battle of Denerim that the Archdemon cannot be killed by anyone but a Grey Warden. In addition, the Grey Warden who kills the Archdemon will die. As the eldest surviving Grey Warden, Riordan volunteers to be the one to slay the Archdemon.

Battle of Denerim
The capital of Denerim is the site of a major darkspawn offensive and this is where The Warden will fight the Archdemon, with the help of the allies gained by using the Grey Warden treaties. At first there is no real sight of the Archdemon until it is seen flying over the city followed by a charge of human soldiers.

The next sighting of the Archdemon is at the Alienage, as the Warden and allies take their leave of the area after saving the Alienage (or possibly leaving it to a grisly fate) and disposing of the Hurlock General there, the Archdemon will make an appearance - flying overhead and destroying the bridge making it impossible for the group to turn back.

The remnants of whatever troops Riordan had led into the area dealing with darkspawn, Riordan himself will be atop a tower where he ends the life of a darkspawn before the Archdemon takes notice of his presence. The Archdemon swoops past the tower and Riordan jumps onto the Archdemon's back and keeping himself on board as it flies through the air by stabbing his sword into its back. The Archdemon struggles to be rid of Riordan it eventually tosses him off, but in the throw Riordan pierces the Archdemon's wing and slices through it as he falls off to his death. The Archdemon however suffers from the blow and crashes into Fort Drakon where it awaits the Warden while it slaughters whatever soldiers are stationed there.

As the Warden reaches the top of the fortress. The Warden is greeted by the sight of the Archdemon making quick work of some soldiers, it's at last time for the final battle...

Tactics
Archdemon has some powerful abilities, such as cleave in front of him (from back when he turns), fire AOE and normal attack, which can knock back his focus target. Choosing the Dalish elves (who use ranged attack) as the support army can be useful, because melee will suffer much more damage from his cleaves, and mages' their AOE spells will affect your melees as well.

Phase One
The Archdemon flies from place to place on the main platform. It is easily attackable but will use various knockback, wing buffet, grab and other attacks. If Shale is in party, turn on crushing blows, this will by default slam as often as she can. Mages who have the spell Inferno should use it as much as possible on the Archdemon. If using the easy level of difficulty friendly fire from the spell won't affect the party.

Phase Two
This begins at 60-40%. The Archdemon flies to one of two high platforms that isn't accessible. If elven archers are available, now is the time to use them as their bows can still hit the Archdemon. Darkspawn appear at the doors. A mage can attack darkspawn with fireballs, but there are a few alphas and shrieks which have normal hit points. During this phase Knight-Commander Greagoir, or First Enchanter Irving will appear and help fight the odds - if you have spare healing use them as they are both valuable allies. The Archdemon is still in range of ranged weapons. Even if not on easy Inferno is a useful spell to hit it with during this phase as the fire won't hit any of your own party. Split the party to the two balistae that can easily target the Archdemon. The balistae CAN jam, if a rogue is in the party they can unjam the balistae, should they jam. Otherwise, it's out of commission for the entire fight. If both jam then rely on mage spells and ranged weapons. Here the elves can be the difference between success and failure if you don't have many hostile spells or a rogue to fix jammed balistae. Engage the darkspawn to keep the elves clear. Mages should stick to healing if they don't have strong offensive spells. Fireball doesn't seem to do any damage to the Archdemon but Winter's Grasp and Inferno do.

Phase Three
This phase begins when the Archdemon hits approximately 15%-20%. The Archdemon flies to the upper part of the main platform. Adds continue to spawn from the doors at an increased rate. The Archdemon uses phase 1 attacks on those within range. There are two balista in range of the Archdemon, one that was used in phase two and one directly the opposite side of the Archdemon from the other. Use them until they jam as they can do up to 115 damage per shot. Tanks get stuck in to keep the archdemon busy. Keep an eye on the party in case their tactics allow adds to get aggro, the tanks should concentrate all their efforts on the Archdemon. Rogues should engage the adds to keep them off the tanks whenever they aren't needed to unjam balistae. It should not take too long to drop the Archdemon if you get this far, but stage three is the hardest to keep track of since you may be firing and repairing balistae, healing and making sure your tanks don't forget what they're supposed to be tanking.

NOTE: Phase 3 can actually be avoided if the Archdemon is killed quick enough - it has nothing to do with it's health so much as how fast you can deal damage.

Another strategy is to send in an army (preferably an up close one) to distract the Archdemon. While your army is doing that you need to get to the nearest Balista and get it ready. When it is ready fire it, and continue to do so. The Archdemon has a little reaction when it is shot which makes it incapable of attacking or moving, before it finishes this motion the Balista will be reloaded. This is more of a glitch than anything and will take you about 10 minutes, however you will get through taking minimum damage. The Archdemon, while incapable of moving, will teleport to other locations once it has taken a set amount of health. Just ignore it as your armies are dealing with it, and run for the nearest Balista to it. Just do the same again, it is incapable of doing anything once you start firing three Balista at it.