Use Igni to set the corpses on fire
In order to start this quest you must find the Priest of the Eternal Fire east from Mulbrydale village (M2,5). During the conversation Geralt will be asked to burn the bodies in three different places. You must accept this task. You can reach the bodies in any order you want. There is a group of ghouls and exploding rotfiends near each of them. After securing a location, you must walk towards the body pyre, spill the oil and use Igni sign in order to set it on fire (9 experience points). At one of the piles Geralt will meet a survivor. Talk to him and you will learn about the true intentions of the Priest of the Eternal Fire (22 experience points).
After burning all the bodies, go North. The Priest of the Eternal Fire has moved there. In your last meeting you can act in two ways. If you accept the payment, you will receive 60 crowns and 45 experience points. If you decline on the bribe, a battle with the priest and few guards will start. It’s not hard to win it. After the battle you will receive 45 experience points. Loot the corpse of the priest – you will find 200 crowns.