You’ll find the Ruinous Shield inside a chest in the Cave to Yet Another World, the last bonus dungeon in the game. This cursed shield can’t be found or bought anywhere else in the game.

Preparations

• Be around level 45. The difficulty is not much harder than in the previous dungeon. If you could beat the Cave to Another world, you’re at the right level.
• Gear Up. Get the Okeanos Sword, Aeras Shield, Pyros Helm and the Gaia Armor by exploring the Cathedral of Blight. Buy the best equipment available at the Divine Shrine too.
• Choose the right vocations. You need someone with a cheap healing skill for this dungeon because you’ll need to heal everyone after each fight. A sage, a druid, or a luminary are ideal.
• Have one Yggdrasil Leaf on every party member. In this way, you can revive anyone during an emergency. 

Finding the Ruinous Shield

Like the previous bonus dungeon, the Cave to Yet Another World is a mishmash of previous dungeons you’ve encountered throughout the game. Once you activate the pillar in the basement of the Divine Shrine, enter the double door in front of you.

1st Floor

Just walk straight north and you’ll find the stairs that go to the next floor. You can enter the temple behind, but there’s only a chest with a Pandora’s Box enemy.

2nd Floor

This floor is almost the same as the labyrinth in the Likeness of the Great Spirit. To reach the exit go through the doors in the following order: west, north, north, west, north, west, west, and follow south until you see the exit.

3rd Floor

Follow the pathway south. Ignore the first intersection, and turn left on the second one.

4th Floor

Walk a bit to the east and then follow south. Open the chest on the way to get the Goddess Ring. This accessory is perfect for healers or spellcasters, as it restores 1MP for every 2 steps you give.

Keep walking south and you’ll see the next stairs.

5th Floor

Turn south on the first bifurcation. Turn east on the next one and get to the dead end. Ignore the first chest (it has a Pandora’s Box enemy) and open the second one to get the Ruinous Shield.

The Ruinous Shield is yours! This cursed shield can only be worn by Aishe and the Hero, and like all cursed gear, it can’t be unequipped unless you go to a church and pay for a benediction.

Even if it has one of the highest defense values in the game, it gives you a +50 to all spells except Zap. Because of this, there are much better shields at this point of the game, making it more of a collector’s item.