Players can be rewarded with some good early summon Materia in Final Fantays 7 Rebirth if they meet some simple requirements. Here is how you can obtain the Leviathan Materia in the game without spending too many hours in the game.
How to Unlock the Leviathan Materia in Final Fantasy VII Rebirth
Leviathan can be obtained in Final Fantasy 7 Rebirth by having a save file from the previous entry Final Fantasy 7 Remake. After booting up Rebirth, head to the Bonuses option on the main menu, then select “Check Saved Data”. The game will inspect your console looking for save files for the Remake base game, its Intergrade DLC chapter, and the FF7 Rebirth Demo.
Having a base game Remake file will grant you the Leviathan Materia, available for use after you get back to playing as Cloud in the present once you complete/skip the Nibelheim incident flashback portion.
Equipping the Leviathan Materia allows you to summon the mighty lord of the seas to help you in battle. He will periodically attack your enemies with Water-based attacks, unleashing its Tidal Wave finishing attack when its summon time is over.
While you only need to have played (not completed) the base Remake game, bringing over your save file from the Intergrade chapter will also unlock Ramuh, which works pretty much the same but has its signature Thunder (and beard) spells.
Similar to the other starting Summon Materias and those obtained through DLC, Leviathan cannot be leveled past 1, so you’re stuck with it like that for the remainder of the game. But it should be strong enough to get you past the early stages.
There is currently no other method to obtain Leviathan in-game without having played Remake, so if you somehow haven’t experienced the first chapter yet, you’re out of luck if you want to conquer the seas with the mighty Leviathan. And if you’re for some strange reason jumping directly on Rebirth without going through the previous entry, you’ll probably feel more lost than you should.