Diablo 1 Walkthrough -> The Defiler
The Defiler is located on the fourth and final level of the Hives. He's the most important figure here and the progenitor of all the flora here. Once you kill him, he'll drop a Cathedral Map. It will show you what you need to do to unlock the path to the next levels.
The map shows that there's a small grave near the Church in the city. You'll see it once you get there. Drop the map on the grave, and a passage will open down to the Crypt.
Source: hermitum.narod.ru.
See also:
Diablo: Hellfire - Little Girl