A lot of this is revealed as the game progresses.
Beware spoilers, but I kept things as vague as I could.
Who or what is Midgardsormr (in game, not the real world name roots):
He is an ancient dragon from an entirely different planet. For reasons not entirely clear, he fled his home world with a clutch of eggs called the First Brood. This brood includes famous dragons you may have met such as Nidhogg.
Midgardsormr made an agreement to protect Hydaelyn in exchange for being able to live on the planet. His body was destroyed in a battle that destroyed Mor Dhona as we know it. His corpse can be seen wrapped around a massive airship in Silvertear lake.
You will learn more of his origins in the raid series for 4.0, but it’s not a lot of details.
The dragons overall story is of course in 3.0, but dragons/dragon related items are very central through the story up to the current patch.
Is Midgardsormr dead? (3.0 spoilers)
No, but it’s complicated. Elder dragons are complicated in this game. They are something akin to primals, but not primals. Their power seems to exist and persist beyond their bodies. Midgardsormr joins your group as a little guy; Nidhogg can reform using the body of another. Dragon blood itself seems to cause people to mutate.
Ascians? (Spoilers but minor)
The Ascians are major enemies in this game. The scene you are talking about with Moenbryda is a start of something bigger: team Scion has found a means to kill Ascians. The nature of the Ascians and what this means will become clearer as youmprogress. Short version: This will become a big deal.