Take his parchment, and read it at the altar of Shadra'Alor, deep within the witherbark realm in the Hinterlands. This will summon Shadra the spider god.
As of patch 3.0.9, if you don't have the Gadrin's Parchment item anymore(it is consumed during the spider summoning), you have to drop the quest and restart it at Master Gadrin again.