Drop-In Side Quest for Any Level
The party finds a secret chamber in a cavern, which was used as a gnoll hideout years ago before being cleared out by other adventurers. The secret chamber features a shrine to the demon lord Yeenoghu. If a PC puts their blood in the bowl of the shrine, they will slowly begin to transform into a gnoll.
This quick side quest can be dropped in any adventure, and can be used at any level. The first section should be used in an underground setting, but can be adapted to be used in a dungeon or other appropriate setting. The consequences of that encounter will develop slowly over the following days/weeks/months.