It is another quest which you will receive after completing the Scenes From a Marriage. First go back to the inn in Oxenfurt. From there you must walk quite far to the south, to the meeting place in the old temple near the Todeas village. At the same time you will receive a side quest (described at the end of this chapter) about meeting with Shani in her house (M1,2) - completing it is required for unlocking the alternative (good) ending.
Go to the old temple. You will get there through the passage through the cave near Toderas village. There you will complete the last Olgierd's objective and will reach the ending. If you have completed the side quest of meeting with Shani, you will have a choice: pick the side of Gaunter O'Dimm's or Olgierd's. If you haven't completed the quest, the battle will be automatically won by O'Dimm.
If you haven't completed the side quest or you picked O'Dimm's side, you will receive a reward of your choosing from him (and some experience):
- To be rich - you will receive 5000 crowns,
- To never feel hungry - you will receive the Horn of Plenty which you can use as an infinite source of food,
- To possess a bottle of liquor that is never empty - you will receive the Bottomless Carafe. Sadly, it has no actual value, you cannot dismantle it or use it,
- To be fast as wind - you will receive the Caparison of Lament which gives 100 durability points for your horse,
- Refuse taking any reward - you will receive more experience.
However, if you have completed the side quest and you know how to defeat Gaunter, you can interrupt him in attacking Olgierd. In that case, you will be transported to another world where you must find the solution to a riddle before the time runs out. In other words, you must quickly reach the mansion at the hill. You can try other solutions on your way, but they are only a waste of time. You will also find some phantoms here which are a reflection of various monsters.
Turn right on the first crossroad, bypassing two enemies (probably the phantoms of drowners). On the next crossroad turn left and, after walking under the rocks, again left. After a while you will be attacked by a Leshen phantom - you can bypass it as well. Notice the red beam of light on the left side.
On the small rock you will find a sword in the rock. It is a good idea to pick it up as it's a unique sword from the Serpent School Gear - the Viper Venomous Silver Sword. Keep running through the road towards the mansion where you will face two more phantoms of gargoyles - you can avoid fighting with them as well. Enter the house. The ground of the corridor will collapse and you will fall down.
Once you're in the ruins, turn left immediately - you will notice one of the mirrors and a broken fountain there. Use the Witcher senses to examine the wall behind it and you will find the riddle. In order to finish this phase, simply destroy the wall with Aard sign.
As reward for helping in defeating O'Dimm, Olgierd will give you one of the best swords in the game - Iris' saber. No matter which ending you have chosen, you will obtain the Pacta Sunt Servanda achievement/trophy.
Optional objective: Meet with Shani
During this quest, before going to the meeting in the temple you can meet with Shani in her house. Completing this objective will allow you to unlock an alternative (better) ending of the storyline. First follow the girl to the door leading to the academy. There you will have to use the longer road. Jump from the bridge and get under the wall. Use the rope dropped by the girl to get inside. However, first get rid of the drowners (level 34).
Once you walk up, you will have to defeat four Witch hunters. Then, walk to the professor's door. You will find out they are closed. The scaffolding can be found nearby, it will lead you to the balcony of the building. Get down to the ground floor and talk with the professor. The key found near him will open the front door.
To get out of the location you must use the small tower near the wall. Walk up the ladder and jump to the water. In order to get to the land quickly, turn south. You will find a boat there. Then return to the main part of the quest.