The Ouroboros Ark is an ark on which the entirety of the human race had been condensed as computer programs to finally reach eternity. It started the entirety of the Epic of Zektbach's storyline by "falling" on the land of Aria Te'Laria, allowing its inhabitants to discover and retrieve Ristaccia ; its remains lie in the Skyship Ruins. In reality, the fall of the Ark was just what happened from the inhabitants' point of view ; what "actually" happened was all caused by Joah, who wasn't satisfied with eternal life as it required humans to give up their personalities, emotions and history, essentially becoming soulless. He consequently created L'erisia as the embodiment of all of these losts elements of humanity and "reset" the species, recreating the entire world and turning it into Aria Te'Laria.
Joah's memos are still present in the remains of the Ark, but as they are a very dangerous truth to L'erisia, she prevents anyone from reaching them - as was the case with Lien, who discovered them and died at the hands of Shamshir. As for the remains themselves, they are known as the "Skyship Ruins" or "Ristaccia Ruins", and many events take place inside them - such as Malchut's defeat, or the final showdown.