Thanks to everyone, I have thought about it and I like the Ritual needing:
{Random Amount} of Powdered Silver (most Divine powers do) the only mine in the area is a Dwarven mine that has recently been overrun by {Random Enemy 34}.
{Random Amount} of Sacred Ash from a burned plant creature, the Wood Elves can help them locate it but they respect all Plant life and must be 1) Bargained with, a Diplomacy check, and then aided in the collection, pacification and purification of the creature.
Blood of a random creature (maybe the rivals have annihilated all creatures in the nearby area, especially if they are rare) which must be tracked (I like having a Ranger in the party) and defeated and it's blood collected to be purified at the church later.
Some other unique item which has to be located, I'm guessing some kind of Relic of the Church.
The rivals could be either a rival church, inhumans who want the ritual scuppered or another band of adventurers hired by the church to complete the same assignment (but I doubt they would destroy all of the creatures for the blood)
any ideas, improvements or suggestions are welcome
EDIT: I have just checked and it seems only Curse and Bless Water need powdered silver, 5 pounds, but the ritual needs a circle of it so they will need a lot of it and it will need to be purified
