A high-level encounter with an ancient dracolich awaits beneath the shifting desert sands!
This is an encounter for players around Level 16, though you can easily scale it up or down. The party stumbles on a lost city, still haunted by the undead dragonrider that once ruled there.
It's one of the few encounters I've written for higher-level parties, and is set in a distant desert in the land of Tarkesh. As you'll see when you read it, however, it wouldn't be hard to adapt the core concept to other party levels and to other biomes. The dracolich boss is based off an adult dragon, but you could make it a young dragon, and reduce the number of ghoul minions to make it easier. Likewise, you might replace the desert setting with a coastal region (the city appears from beneath the waves) or a snowy mountain range (the city was frozen in a glacier), and so on.
Tarkesh is part of my growing "shared universe" of campaign settings, and can be found on the same continent as Ward (from The Griffin Conspiracy) and Miskachuan (where New Arkon is based).
Enjoy!