The Crypt of St. Bethesda (2000)

From the front cover:

Out of Body, Out of Mind is a d20 System adventure booster designed for 5-8 characters levels 2-4. It can be played as a stand-alone adventure or dropped into any ongoing campaign setting.

From the back cover:

Not all threats lurk in the wilderness, and not all dungeons lie far from the realms of man. The gravest threats appear in the heart of civilization, hiding beneath harmless veneers and waiting for the moment to strike.

You and your friends find yourself in a large and prosperous city... Your path takes you through a dark area of the city: the cemetary district, where fields of graves jostle against ancient churches and family crypts. Off in the distance somewhere, you hear a scream, but there's no way to tell where it came from, so you ignore it. Rounding a corner soon after, one of you stumbles over something in the dark.

At first, you think it's just a pile of garbage, but a closer inspection reveals something much grislier: a human corpse, its face frozen in a rictus of fear. Several stab marks ooze fresh blood from its belly, and its flesh is marred by what appear to be acid stains. The skin is warm to the touch and some fo the injuries still smoke. He's not been dead more than a few minutes....

"Halt!" The clink of mail, and the sound of running feet break up your examination. A squad of city guardsman--several blocks away but closing fast--flash their lanterns in your direction.

"Murderers!" the cry goes up. "Halt in the name of the watch!"

This doesn't look good.