AD&D: U1 - THE SINISTER SECRET OF SALTMARSH (1981)

From the title page:

Desolate and abandoned, the evil alchemist's mansion stands alone on the cliff, looking out towards the sea. Mysterious lights and ghostly hauntings have kept away the people of Saltmarsh, despite rumors of a fabulous forgotten treasure. What is it's sinister secret?

The Sinister Secret of Saltmarsh is the first installment in a series of three modules designed and developed in the United Kingdom, for beginning adventurers with the AD&D(TM) rules. The adventure can be played by 5-10 characters of level 1-3. This module contains large-scale maps, full background information and detailed encounter descriptions for the players and DM.