Baldur's Gate 3

This is a main quest achievement tied to the final battle of the game against the Netherbrain. You are not required to have any specific companions in your party and will always be given the option to select the necessary dialogue no matter what choices you have made throughout the game.

After successfully defeating the brain, you will be presented with 3 dialogue options:
1) Go ahead. Command the brain to destroy all tadpoles and then itself.
2) Let's dominate the brain and rule the world, together.
3) Dominate the brain. Become the Absolute. (this may say "Betray Orpheus/the Emperor" as well if you have one of them on your team)
CREATE A SAVE WHEN YOU SEE THESE OPTIONS ON SCREEN! You can reload the save to get the other brain related achievements!

For this achievement, you are required to select Option 3: Dominate the brain. You will be met with the credits and your achievement after the domination cutscene plays out.

You can then reload your save to select Option 1 and get the "Hero of the Forgotten Realms" achievement.

This is tied in with “The Wizard of Waterdeep”.

"The Wizard of Waterdeep" objective should be Claim the Crown of Karsus. After you take the boat under the city you you'll get to a point in the story where the emperor says he wants to absorb Orpheus, you can either let him or become a Mind Flayer and consume Orpheus yourself.

Continue until you get to the fight with the netherbrain and once defeated, you should be presented with a dialogue option to either betray the Emperor and/or (if you ate Orpheus yourself) dominate the brain. The achievement should pop during the cinematic.