The cave for this achievement (Mysterious Cave/Morals Cave) is located in the Orobas Fjords by Champion Harbour. It's location 5.10 on this map:
http://guides.gamepressure.com/divinityiiegodraconis/guide.a...For this, you just need to make all 3 good choices or all 3 evil, like Malholio stated!
There are 3 encounters that I'll show you both of the choices with their respective dialogue cues. Also, if you pick the bad option as your first choice, the situations for 2 and 3 will be different (thanks to Trombonafide for pointing that out)
Below is the GOOD path:1) A guy named Kantar is trying to take the gold from a princess named Merela.
GOOD: Side with Merela - "Step aside, scoundrel, lest you want to invoke the wrath of a Dragon Knight!
BAD: Side with Kantar - "No dice, toots! Me and scallywag here are going to rob you and share the profit."
2) Elaine is pleading her lover, Peppin not to go to war because he is a farmer and not a soldier. She then asks you to teach him how to fight because he is insistent on going.
GOOD: Give Peppin some of your health so he can survive longer - "I know of something that will help. *Magically transfer one of your stat points to Peppin.*"
BAD: Let him leave without helping him - "Off you go then! And don't worry, I'll be here to comfort your widow - er...wife."
3) Geoffrey is laying on his deathbed and wishes to see his son one last time before he dies. However, his sons journey is a long one and he will die soon so he asks for two more days to live.
GOOD: Help him live for two more days - "Two days you need and two days I shall grant you! *Permanently transfer one vitality and one spirit point to Geoffrey.*"
BAD: Don't help him - "Very sad. Well, must be off. Have a nice evening, then!"
Below is the BAD path, the first choice is the same in both situations, just make sure you pick all good or all bad options (again, thanks to Trombonafide):
2) Cano and Mono have a woman trapped in a cage. They walk away, leaving the key in plain sight.
GOOD: Free her - "Of course. Here you go!"
BAD: Leave her - "No, I think I'll use this opportunity to help myself to their riches!"
3) A group of people are tied to stakes, awaiting death.
GOOD: Pull the lever marked "Life"
BAD: Pull the lever marked "Death"
Once you make the final choice as all bad or all good, your achievement should pop. Also, you get a reward that is dependent on your choices:
GOOD REWARD: Sword of the Paladin/Bow of the Paladin/Warhammer of the Paladin
BAD REWARD: Sword of the Damned One/Bow of the Damned One/Warhammer of the Damned One