You must have finished the Truth Beyond the Gate quest and entered Pagan Temple gates at least once you need to have the Faded Visitors Mark in inventory.
1. Talk to priest Flauron on the upper right side ramp of the Einhasad temple in Rune (right side hallway upwards in front of the entrance to the temple on ground level)
2. He is a bit stubborn so you have to argue with him a bit. Finally he agrees to make a deal.
3. He sends you to Forest of the Dead to kill the necromancers and get 10 hearts.
4. They drop relatively fast so after you have got the hearts, return to Flauron and he gives you the Pagans Mark. He speaks a lot but pay attention to what he says about big parties might come handy