The avowed of Candlekeep have learned of the possible location of a one of the lost Nether Scrolls. A work of the ancient Netherese that is said to bestow great power upon any who study it's writing. There have been attempts in the past to retrieve it; all have failed. Now, they hope that your party will travel into the desert to retrieve the scroll and bring it to Candlekeep where it can be safely stored less it one day falls into the wrong hands.
This adventure is really fun, but there are a few possible improvements. Instead of a random encounter with an androsphinx, have it acting as the guardian to the caves where they must get past it to enter. Similarly, you could have the ancient dragon tortoise have knowledge of the lost city that they can impart on the characters if they are helped.
Lastly, I wasn't happy with the amount of space for the dracolich fight, so I have included a map with the roof of the tomb torn open so as to allow the fight over a larger area. If you use this, then I would recommend that after the first player goes, the dracolich will use it's legendary action to do a wing attack and fly through the ceiling causing a dc 15 dex save to avoid taking 2d10 (or 4d10 if it's a strong party) bludgeoning damage and becoming restrained under rubble.
These are just some minor thoughts to help the story feel a bit more cohesive instead of just some random encounters that don't connect to the rest of the adventure that is going on.
I hope your players have fun!