Demons Below?

From publisher blurb: Hideous creatures, suspected to be demons, have been spotted in the countryside, and the adventuring party—shall we call them heroes?—has been tasked with finding the source of the demonic incursion and putting an end to it before bad things start happening. Because no one wants that, right? This D&D 5e adventure is designed for a party of four PCs levels 1 through 4.