For Rare Nevi location, see The Ruins: Rare Nevi.
Kat first visits The Ruins during the mission "The Lost City" while she's in Auldnoir. Upon meeting up with Gade, he informs her about how the universe has taken the homes and families away of the residents within the town and how they're waiting for the universe to return back to them what's rightfully theirs. When he notices Kat's interest in wanting to help the townsfolk, he proceeds to bring her to another world known as The Ruins.
The master of this area is the Taion Nevi.
Within The Ruins, Kat comes across the first Power Tree and learns her first special attack, Spiraling Claw.
In conjunction with defeating the nevi so that the Rift Flowers can bloom and guide her, Kat makes her way through the rift plane mainly by shifting gravity to isolated terrain structures—specifically the pieces the Nevi and rift flowers are on—in the air which is due to the lack of interconnected terrain in this rift plane.