We just did this last night with some twists.
Earlier in my campaign before the Githyanki invasion, the characters stumbled though the portal in the Black Spine mountains, did just a touch of planar exploration, Ended up in the realm of Wee Jas, one of the characters was selected to become her follower, and her champion for this world called Athas. The same character also got Scorcher.
So when it came down to the Dregoth fight, my player spent 40 minutes stumped on a decision. Help stop Dregoth or Help defend Dregoth so Wee Jas can get a foothold on the world.
He decided to stick it out with his adventuring companions to stop Dregoth.
The real boy NPC that had been with them for 5 months now was One Word Killed.
At the end of the fight, I had Dregoth use time stop, take 2 full attack rounds on the character with Scorcher, knocking him for dead, stole Scorcher and disappeared.
This was the end of our campaign, now the world is left with no good connection to the Inner Planes either. What to do.