Midgardsormr, also known as the Guardian of Silvertear Falls (and sometimes incorrectly referred to as the Keeper of the Lake) was a great dragon that lived in Mor Dhona from primeval times until his death in 1562 (Sixth Astral Era) in the Battle of Silvertear Skies at the hands of the Garlean Empire's massive airship, Agrius.
The legend, as told by Erik, states that, "There was the birth of Althyk, god of time among the Twelve, and in turn, His younger sister Nymeia, goddess of fate. And then there was water, and through it the Silvertear Falls came to be at the center of all that was. Here was the source not only of water, but the fount of all magic as well. Now, when water came into being, so too did the great dragon Midgardsormr. Brother Time and Sister Fate, fearing the Falls might fall into the hands of evil, ordered Midgardsormr their protector and warden. Much later, with the arrival of man, would Midgardsormr be worshipped as the guardian deity of Silvertear Falls."
When the Empire invaded Mor Dhona, knowing it to possess Eorzea's greatest concentration of aether, they incurred the wrath of Midgardsormr, who burst forth from the water to fulfill his purpose as its guardian. Despite fighting "with divine strength and purpose," he, and the dragons that came from Dravania to fight alongside one they revered as a king, could not overpower the might of the Empire's armada.
His last act was a crippling blow to Agrius; enraged, Midgardsormr coiled around the airship and broke through its hull with a massive energy blast. However, as Agrius crashed and Midgardsormr let out a victorious cry, the ceruleum fuel stores within the ship ignited and the resulting explosion destroyed them both. The devastation was such that the Empire retreated to Ala Mhigo and dared not enter Eorzea again until the VIIth Legion under Nael van Darnus was ordered to the western front in 1571.
The remains of Midgardsormr and Agrius still sit in Mor Dhona today, where together they form a monument known as the Keeper of the Lake, and stand as what some consider proof of the existence of the gods.