Corypheus

"The city! It was supposed to be golden! It was supposed to be ours!"

Corypheus is an ancient darkspawn trapped inside a prison that was built by Grey Wardens in the Vimmark Mountains.

Involvement
Corypheus is the main antagonist in the Legacy DLC for Dragon Age II. He is the final boss of the Legacy (quest). The Grey Warden Janeka wants to utilize him as a weapon to combat the Blight, while the former Warden-Commander of the prison, Larius, wishes to kill him before he escapes the prison due to the weakening seals.

Tactics
Corypheus is one of the most difficult bosses in Dragon Age II and you will need to carry enough potions for this battle. The battle consists of two phases that switches between them when the boss' health reaches 75,50 and 25%. In the first phase, Corypheus will teleport around the battlefield and start dealing moderate ranged damage to random party members, much like the Ancient Rock Wraith. His melee attack is powerful and his attack will damage and knockback anyone within the sweeping radius. If engaged in melee he will generate a bubble that knockbacks and dispels any effect on him.

Phase two starts when Corypheus is down to 75%, after that he will teleport to the middle of the room and start channeling two rotating walls of fire, in front and behind him, while drawing power from the 4 pillars in the room. In this phase you will have to interact with each pillar, at which moment it will summon two shades that you will have to defeat, but with a 90 elemental damage bonus from the pillar, depending on which you just touched (Nature, Fire, Lighting, Cold).

Corypheus will create a maze at the 50% health mark, and start channeling the walls of fire. Bear in mind that the maze will remain placed during the rest of the fight. At the third phase switch(25% of his health remaining), he will create a thunderstorm and start summoning falling icicles, making an ice patch in the place that they'll fall and slowing anyone damaged by it. The lightning will close any passage through the maze and deal a moderate amount of damage to anyone who pass. The pillars will now spawn two lieutenant ranked shades, so they will have to be dealt with quickly to avoid being damaged by the fire. When the pillars are turned off, blow all cooldowns on Corypheus while avoiding the icycles and lightning when he teleports.

Having Rune of Fire Warding is very useful, as the walls deals heavy damage even with them, and will help in resisting the balls of fire he throws during phase one. The shades are best handled in the same place they appear, as the wall will do less damage while being on the stairs of the niches. Also, for an unknown reason, during phase two you will be randomly healed while fighting the shades, probably because of the effects of the pillars while being near them.

Quotes

 * "Be this some dream I wake from? Am I in dwarven lands? Why seem their roads so empty?"
 * "You! Serve you at the temple of Dumat? Bring me hence! I must speak with the first acolyte!"
 * "You look human. Are you not citizens of the Empire? Slaves then, to the dwarves? Why come you here?"
 * "Whoever you be, you owe fealty to any magister of Tevinter. On your knees! All of you!"
 * "Dumat! Lord! Tell me. What waking dream is this?"
 * "The light. We sought the golden light. You offered... the power of the gods themselves. But it was... black... corrupt. Darkness... ever since. How long?"
 * "If I cannot leave with you, I will leave through you! I seek the light!"
 * "Dumat! Grant me your power!"
 * "Dumat! We made the sacrifices! Give me strength!"

Trivia

 * The achievement "Conductor" requires the player to defeat Corypheus.
 * Even though he is asleep he is able to communicate with those who have the Darkspawn taint including Grey Wardens. This causes Larius and Anders great distress.
 * It is possible that, like the Archdemons, Corypheus is able to transfer his soul to the nearest organism bearing the Taint and survive destruction if killed by a non-Warden. This would explain how he might have survived the events at the end of Legacy.