The Cathedral Map is a map in Diablo: Hellfire, dropped by The Defiler, who resides on the fourth level of the Festering Nest. When you have this map in your possession you can talk to Gillian to initiate the Grave Matters quest.
The player can use the map to get an overview of the town of Tristram, the way it looked a long time ago. The map itself is used to open up a way to enter the Demon Crypts by dropping it on the large ornate grave in the church graveyard.
The map's artwork (as an inventory item) is leftover from a quest that involved reaching and slaying Diablo quickly before he gains greater strength. The item was supposed to be a star map that indicated how much time is left.
The artwork seen when the map is used is most likely a leftover from the "buried treasure" quest, which also never made it into the final version of the game.