Leviathan

Leviathan swallows your ship on the way to the Mysidian Tower. This one-time dungeon has 5 permanently missable chests (including the Gaia Blade), 4 new bestiary entries, and one boss.

From Cave of Mysidia, sail north toward the Mysidian Tower. A cutscene triggers when you get close; there's no way to avoid it.

Inside Leviathan

The stomach acid (orange liquid) on every floor deals 1 HP to each party member per step. Not dangerous on its own, but it adds up across three floors. Bolt Fish (540 HP, 60 ATK) and Manta Rays (870 HP, 70 ATK) are the main encounters inside; Sea Dragons (870 HP, 70 ATK) appear on the later floors and hit just as hard. Keep HP topped off between fights.

1F. The dungeon is completely linear. Go west through the acid and follow the path. Chest 1 (Twist Headband) is along the way. Continue through more acid to Chest 2 (Power Sash) near the exit, then head through the door to the next floor.

2F. No random encounters on this floor. Head north through the acid pool and west to find Ricard, a Dragoon. Show him the Crystal Rod and he joins your party. Go north of him for Chest 3 (Power Armlet), then continue to the top-left exit.

3F. Chest 4 (Gaia Blade) is at the top of the map; it's a solid weapon upgrade for your strongest physical attacker. Chest 5 (Diamond Shield) is on the south side, guarded by 5 Red Souls. And once you've grabbed both chests, head to the middle-left of the room for the boss.

Roundworm

Cast Blink and Protect before engaging. Roundworm's 120 ATK hits are among the hardest in the game so far, but it only uses physical attacks. Defensive magic shuts it down completely; without buffs, those hits will drop party members in two rounds. Dedicate one character to healing and keep Blink images refreshed on your frontline fighters.

Board the ship after the fight. Ricard stays as your fourth member; he's a solid physical attacker who comes with decent starting gear.

Head to the Mysidian Tower next. The Crystal Rod opens the entrance, and the Ultima Tome waits at the top. The tower is the longest dungeon yet with 10 floors; stock up on Ethers and make sure your healing magic is leveled before going in.