18. Castle Hein

This dungeon is a one-time visit, make sure that you collect all monsters here for bestiary.

Living Woods - Missable Achievement (Optional)

Achievement: Adventurous Wayfarer
This area won't be available after the Castle Hein event.

Hop on your ship and follow the coastline south until you see an opening between the mountains into the woods.

Just like Chocobo Woods, only Fairies live here. However, they provide some clues on how to defeat the upcoming boss when you interact with them.

It's also required to complete the Adventurous Wayfarer achievement.
Next, head back to Tokkul to continue the story.

Tokkul

Now head back to Tokkul. Once you enter the town, you won't be able to come out before you complete the event of Castle Hein. I recommend that you save your game upon entering.

Castle Hein

Kill these monsters before you complete the dungeon:

[FFIIIPR-066]066. Pharaoh
[FFIIIPR-067]067. Lemur
[FFIIIPR-068]068. Lamia - rare encounter, I found her in the room just before the boss.
[FFIIIPR-069]069. Demon
[FFIIIPR-070]070. Dullahan - 3F, rare encounter.
[FFIIIPR-203]203. Hein - Boss

You'll reappear in a prison along with King Argus. There is a pot in the bottom-right corner which restores HP/MP and revives KO'd allies.

Talk to the soldier at the left wall to receive Mini.
Cast Mini on everyone, then examine the small hole at the top-left corner. Then re-cast Mini to remove the effect.
Follow the map and collect all chests.

Boss: Hein

The battle starts with the boss having no weakness. Once he uses Barrier Shift, he'll gain one elemental weakness, so having a Scholar in your team is a good strategy.

Use the Scholar's Study command to scan for his weakness. If you follow this guide, you should also have Libra on your White Mage, which works the same as Study.
Once you know his weakness, use the elemental magic he is weak against. Black Mage can use elemental spells, and others can use magic items such as Bomb Fragment, Antarctic Wind, or Zeus's Wrath.
Bonus: Having a Scholar use magic items (Alchemy command) during battle will increase their potency.

Chests/Items:

I-036: Mini - 1F, soldier in the prison cell.
C-135: Holy Arrow - 2F, bottom chest.
C-136: Phoenix Down - 2F, near the exit after the hidden passage.
C-137: 30 gil - 3F (1), left room, left chest.
C-138: 100 gil - 3F (1), left room, right chest.
C-139: 3300 gil - 3F (1), middle room, left chest.
C-140: 3000 gil - 3F (1), middle room, right chest.
C-141: Phoenix Down - 3F (1), right room, left chest.
C-142: Zeus's Wrath - 3F (1), right room, right chest.
C-143: Royal Sword - 3F (2), near the entrance door.
C-144: Phoenix Down - 4F, east block.
C-145: Elixir - 5F, top-right corner.

Living Woods

After defeating Hein, you'll reappear outside the Living Woods area.