Diablo Wiki

The Poisoned Water Supply is a sublevel on the second floor of the Cathedral in the original Diablo. It is only accessible when the quest with the same name is available.

The entrance is a hole labeled "Dark Passage", in some wall on Level 2. Inside it looks like the later Cave levels, with boasting considerably easier enemies implying Carvers, Devil Kins, Flesh Clan mace warriors, which can pose a serious threat to most early characters, and one single Flesh Clan archer. Once all monsters inside the level are killed, the poisonous water in the river will turn blue again, so does the water inside the well on Tristram's market. After cleaning the level you can visit Pepin to receive the Ring of Truth.

Level Bestiary

The following monsters can be found in Poisoned Water Supply: