Rise of the Third Power

Assuming you've collected all the relics before finishing the events in Udingrad, you will see a scene after escaping that shows four sets of 4 torches activate. When you regain control of your ship, you may tackle these in any order. I recommend starting with the Island horror because the prize helps with the remaining fights

My party consisted of Corrina, Reyna, and Natasha and my strategy consisted of giving poison to Corrina, Marking the boss, using one Eviscerate with Corrina, and then spamming Coup de Grace as Natasha used rapier to prolong the poison and Reyna healed/shielded/buffed as needed. I recommend equipping Reyna with the Fencer Bracer for the chance for bonus actions

Other party combinations may work better, since you'll have access to team attacks.

The locations:

Island NW of Jena'an (Do this first)

Island South of Angelico

Fort Kahleen (Use Reyna's Dispel at 2 or 3 stacks of Demonic Blessing)

Frozen Forest northwest of Tika Village