The developers of Baldur’s Gate 3 designed most questlines so that they may be completed in multiple ways. Thus, giving the players more freedom in how they approach situations. This guide represents one method that is confirmed to unlock this achievement.
A full achievement guide is available here:
https://tigorestips.com/dnd/baldurs-gate-3-achievements-trop...Saving Gale
Save Gale from a portal on the Ravaged Beach where you land after escaping the ship at the start of the game. Then, he will periodically ask that you give him a magic item that he can drain to satisfy his condition, so keep a few magic items on hand that are not needed. Over time he will reveal more details about what happened to him.
Repairing the Weave
As you travel toward Moonrise Towers via the Mountain Pass, you will come across a weary traveler, Elminster Aumar. Invite him to speak with you at camp and he will give Gale a Charm which will stabilize the Netherse orb. Doing so will unlock the Repairing the Weave achievement.
Destroying the Netherbrain
After defeating Ketheric Thorm at the top of Moonrise Towers, make sure Gale is in your party before entering the Mind Flayer Colony via the tower in the northeast corner of the roof. Head straight toward Ketheric Thorm in the far east end of the colony. During the cutscene, have use the Netherse Orb to end the Netherbrain.
If Gale wasn’t in your party or you missed the opportunity in Act 2, this can also be done at the end of the game. Make sure Gale is in your party when you get in the boat to travel to the Upper City for the final sequence in Act 3, and he will offer to use the Netherse Orb there.