This Seeking Tech guide will explain how you can unlock the eviathan and Ramuh summons in Final Fantasy VII Rebirth.
To do so, you can check out the video and text guide below:
Step 1: Make sure you have save data from both Final Fantasy VII Remake and Final Fantasy VII Remake Episode Intermission stored locally.
Step 2: Go to the title screen of Final Fantasy VII Rebirth.
Step 3: From the title screen, select Bonuses > Check Save Data.
Step 4: Afterwards, load into your current save in Final Fantasy VII Rebirth.
Step 5: Once you load your save, open the main menu and then select System > DLC / Bonuses.
Step 6: Finally, redeem the Leviathan and Ramuh summons.