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 the Emperor" as well if you have the Emperor 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.