Speak with Morrigan
If Morrigan doesn't have the child, skip to the next section.
If Morrigan has the child named Kieran you must chase her through the eluvian into the Fade. Find Morrigan and help her search for Kieran. You don't have to travel far to find him...with another person. You will uncover quite a bit of story between these characters as this all unfolds. If the Inquisitor drank from the Well of Sorrows, Flemeth will instruct you do travel to the Altar of Mythal and summon a Dragon. Subsequently, Morrigan will know what needs to be done if she drank from the Well.
All that remains is to find Corypheus. Summon the War Council when you feel ready.
The Final Piece
Before setting out you want to make sure you prepare for this because you will be fighting a High Dragon. Replenish potions, hand out fire resistance gear and frost staves, throw in some Dragon Runes. If Morrigan was without the child then she comes with you and this is where you will meet Flemeth. Morrigan is not pleased. You are now tasked with summoning a Dragon.
BOSS - Guardian of Mythal
|Resistance||Greater Fire Resistance|
|Immunity||All Disabling Effects, Cold|
This girl is tough but lucky for us we only need to get her down to about half health. She has all the attacks we've seen from other dragons: shoots fireballs, firebreath, claw swipes, tale swipes, and wing flap that pulls you closer. There's basically nowhere to hide either, the terrain is flat. It's a good idea to bring two tanks and two ranged so the tanks can take turns keeping her attention while archers/mages lay into her.
When she takes enough damage she will move but she won't take flight. If she gets close to your ranged characters, move them to a safer distance but watch for those fireballs and the wing flap. When she starts her wing flapping, warriors can sink in and do some good damage while the ranged characters try to jump away and recover. Keep hacking away, use some Replenish Potions, and you'll tame this beast.
At half health a cutscene starts and the Inquisitor is able to stop her from attacking.