It seems quite likely, that the reason for abandoning this quest was, that after integrating the dark mage into the hellfire-story and showing him via the intro-movie, meeting him ingame wasn't necessary anymore: (Hellfire-Story-Excerpt)

"Not long ago, before the betrayal of Archbishop Lazarus and the freeing of Diablo, a Dark Mage was seen on the outskirts of Tristram. Many claimed that he often lurked near the cemetery at night. As this skulking figure did not appear to harm the graves, the townspeople left him alone. But then, one evening, a thin smoke came drifting from the soil, as from a smoldering fire. An orange glow from below lit up the tombstones, and someone thought that he saw the Dark Mage descend into the earth itself. The smoke dissipated, the glow faded, and the cemetery was still. Whispers of evil sorcery and necromancy filled the tavern gossip for a while, but were soon forgotten as King Leoric grew madder and other troubles plagued the town."

Trigger: Most likely the one, who mastered the dark arts himself.
Gist: Obtain the goods from Adria and receive something in return
Reward: Unknown

Additional Information:
-)graphics: the darkmage-monster-appearance may have been used
-) Why does that speech belong to the Dark Mage of the Hellfire-Story:
1)One can only be aware of the weather-situation, if you're standing outside, so why not near the cemetery.
2) He's in need of material for a dark ritual, which does fit into the picture, too.
3) "An orange glow from below lit up the tombstones, and someone thought that he saw the Dark Mage descend into the earth itself." This describes the composition of the Crypt-entrance and that the sorcerer did use it (descend into earth = walk down the stairs [at night + from the distance one wouldn't be able to spot the stairs, right?]). On a final note bats are going to be created via the ritual (2 subspecies of flying horror = firebats & hellbats), which are both to be found in the Crypt-Levels.