You'll earn this after fighting Obsidian, the Evil Spirit of Crimson Forest.
I was about level 20 and about 9 hours in, so It's not very tough. Some tips to help you out:
- Have Kaim and Seth attack Tentacles A, B and D, and Jansen cast Prism. Once Prism is cast, 3 tentacles should die (assuming Kaim, Seth and Jansen are strong enough), this will give you more time to save your restrained party member as Obsidian will only absorb HP once per turn instead of 3. Now use Magic on Tentacle C, physical attacks are very weak against Obsidian.
- If you have it, cast All-Shield and All-Barricade to protect against Shadow and Obsidian Sigh.
- Cast Sleep on the possessed character to attack Obsidian once you kill the Tentacles, not doing so will result in that character getting hit and dying and require you to re-fight the boss. You'll have to cast Sleep every 2-3 turns, so watch out.
- Have your other new party member use Zephyer to heal or maintain Shield and Barricade. They can't do anything else anyways.