You will meet the Emperor while resting in Wyrm’s Lookout on your way to Baldur’s Gate at the end of Act 2. Later, when you enter the Astral Plane, side with the Emperor. When the Emperor offers a new tadpole to help you evolve, agree to it. Later, during a long rest, the Emperor will confide in you about its past. Choose encouraging/positive responses and offer to hold hands.
Once you reach the Lower City, there will be a quest marker on Elfsong Tavern to investigate the basement. Head down there, clearing out the rats and press a button on the back left wall. Then, follow the prompting to go through the large doorway that opens in a cask. Inside you will discover the Emperor’s old hideout. Interact with each of its trinkets. Finally, when you take the next long rest, the Emperor will visit again. If you are encouraging, the Mind Blown achievement will unlock. You can skip the rest of the interaction after it unlocks.