Whatsoever A Man Soweth - Hearts of Stone
This quest automatically begins upon the completion of Scenes From A Marriage, a Hearts of Stone Story Quest. For the first part of this quest you'll need to visit Olgierd who is at the Alchemy Inn at Oxenfurt. When you arrive you'll speak with O'Dimm instead of Olgeird and your quest will update with an additional side objective (that you definitely want to fulfill) for visiting Shani.
If you don't visit Shani and do this optional part of the quest then you'll be unable to get the "good" ending for the Hearts of Stone Expansion. You'll also miss the opportunity to play Shani in Gwent and miss the Olgierd von Everec Card. Visit Shani and then follow her to the academy where you'll get separated from her.
To continue the optional part of this quest you'll need to dive off the bridge and swim around to the side of the academy where there is a pack of 3 Drowned Dead. Shani will lower a rope to let you in after you defeat the Drowned Dead that are here. After a brief scene you'll have to fight 4 Witch Hunters before finally arriving at the professor's front door, which is locked.
In order to gain entry to his house you'll need to climb the ladder which is across from the locked door then go across the wood scaffolding until you reach the balcony (pictured above). You'll have to jump across onto this balcony then use Aard to blast open the door for entry.
You'll have a scene inside of the building after which you'll want to loot the corpse for a key and leave the Academy's grounds by climbing a ladder into a tower near where we climbed up here and jump off it into the water. Before leaving Oxenfurt you'll want to return to Shani's Clinic which is where you'll be able to play her at Gwent and win the unique Olgierd von Everec Card. If you've forgotten her Clinic's location use my screen shot below.
Map location of Shani's Clinic.
Once you're done in Oxenfurt you'll want to travel east of Toderas which is where you'll find the Temple of Lilvani, the place you must meet Olgeird and O'Dimm. When you arrive at the cave entrance it's highly recommended you save your game as the upcoming part of the quest is easy to mess up.
If you took the time to speak with Shani in Oxenfurt then you'll be given an option when O'Dimm is about to take Olgeird's soul; help Olgierd or leave him to his fate. You'll want to choose to help Olgierd, although he probably doesn't deserve our help. However, the main reason you want to help Olgeird is because in the "alternate world" that O'Dimm casts you off to, you'll be able to find a Venomous Viper Silver Sword.
Siding With O'Dimm
This is considered the "bad" ending for the quest and is the default ending unless you talk to Shani in Oxenfurt at the start of this quest to learn how to defeat O'Dimm. For this outcome you'll get 5 different choices - each provides you with a different reward.
1. Wanna be Swift as the wind (100 Stamina Horse Saddle)
2. Never go hungry again
3. A bottle of Vodka that
4. Make me rich
5. Don't want anything
Helping Olgierd
O'Dimm will give Geralt a riddle when he awakens. After that you'll be tasked with finding O'Dimm (who is in the mansion on the opposite side of the area we started at). However on the way to O'Dimm you'll want to stop by one area which is where we'll get our sword.
Note: Don't stop to fight anything in this world nor do you want to stop for anything that isn't mentioned in this guide. Almost everything in here is designed to slow you down on your way to the mansion.
Shortly after crossing the wooden bridge you'll come to a fork in the road, one side with a stone archway (pictured above). Go under this stone archway then at the next fork in the road make another left. In the distance you'll see a red beam, this is where the sword is at. On the path leading to this sword you'll have to fight a Specter which looks and fights very similarly to a Leshen, quickly defeat it, grab the sword then return to the main path.
As previously stated our end destination is the mansion at the end of the path. Inside the mansion you're going to want to find a mirror - which is the answer to O'Dimm's riddle. However, every time you get close to a mirror in this mansion O'Dimm will blow it up, so what we need instead is some water.
In the mansion you'll find a mountain, but no water. Near that fountain you'll find a wall which is blocked by stones (pictured below). Use Aard on this wall to clear the stones and get some water out here, a scene will begin and your quest will complete here.
