The Shrine Alliance is an ancient pact between the various flan gods commonly worshipped in Perrenland.
The alliance was formed over 2000 years ago in the ancient city of Ulmt. Those deities that signed the pact are now members of the Old Kerk. As part of the agreement, Nerull agreed not to turn the living in Perrenland into undead. In return, Nerull would continued to be acknowledged and respected. Although not actively worshiped, Nerull was mostly appeased in the annual ceremony, the Dance of Nerull.
Although specific information is difficult to obtain, it seems that the Shrine Allaince was an attempt to unite the faiths to drive out and destroy a demonic incursion in the land. It appears to have succeeded.