Adventure Hook by Jason Bowers
An acting troupe hires the heroes to search for a genius playwright that went missing. He owes them a long-awaited manuscript of a play. The rumors are that the playwright went mad. As the heroes search for him they find his house overrun with Byakhee.
Once they find the manuscript, they are betrayed by the acting troupe. They are Yellow Sign cultists in disguise that need the manuscript to summon Hastur. As the undead rise from their graves and Byakhee fill the sky, it is up to the heroes to forge an uneasy alliance with a power called Gla’aki, which has monstrous ambitions and minions of its own!