If you own a copy of Open Grave and haven't gone around to using some of the new monsters listed in it, here's a simple side trek suitable for 13th level characters.
The side trek begins when the PCs set up camp for a night and they find two tombstones which indicate two hurriedly made graves belonging to dwarf adventurers. One of the grave is opened and the body missing while the other is closed and intact. Upon closer inspection, the PCs will find cockroaches around the open grave and a track that leads them 3 miles towards a mountain.
If the PCs investigates they would come upon a doorway to a crypt that is blocked by a heavy slab of stone. Once entered, the PCs are jumped by the undead that can be found from the book. When the PCs are done dispatching them, all that's in the crypt is a coffin which is the final resting place of minotaur mummy. The potentials and questions that needs answers is left up to the DM from there and there are suggestions on how to scale the encounter down to 10th level or higher to 17th level.
For those who don't own the book, you can check out the stats for a Scourge of Baphomet Swarm Shifter at the bottom.