Whatsoever a Man Soweth is the fifth and final main quest in The Witcher 3's first DLC expansion, Hearts of Stone. This mission sees you siding with either Olgeird von Everec or Gaunter O'Dimm, with each option offering a set of unique consequences and rewards, such as the Venomous Viper Silver Sword.

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• Optional Objective: Talk to Professor Shakeslock
• The Temple of Lilvani
• Side with Gaunter O'Dimm
• Side with Olgeird von Everec

Return to the Alchemy Inn in Oxenfurt to begin the final quest, and speak to Master Mirror inside the tavern.

Once the conversation has ended, make your way down the block to Shani's Clinic.

After speaking to Shani, decide whether or not to venture to the Oxenfurt Academy to speak with the Professor about Gaunter O'Dimm. You can also head directly to the Temple of Lilvani.

Optional Objective: Talk to Professor Shakeslock

While Shani is permitted to enter the Academy, Geralt will need to find another way in. Turn back and cross the bridge, then head down to the riverbank and swim across the narrow channel to reach the underside of the Academy.

Head around the base of the structure to your right and kill the small group of drowners that are lurking there, then wait for Shani to drop you a rope from the top of the wall.

Once inside, there are four Witch Hunters that you'll need to eliminate before you'll be able to reach the professor's chambers.

After the guards have been dispatched, head up the path under the archway to enter the main courtyard. The door to the professor's house is locked, so you'll need to find another way inside.

Using the ladder across the yard from the locked door, climb up onto the scaffolding then cross over to the roof of Shakeslock's offices. Jump onto the balcony and use Aard to make your way inside.

Head downstairs to talk to the professor. Once the conversation has ended, exit through the front door of the house and use the tower (in the first courtyard you entered into) to leap into the water below and make your way towards the Temple.

Before heading towards the temple, you can visit Shani again for a talk, and also play Gwent with her to win a unique Olgierd von Everec card.

The Temple of Lilvani

Make your way to the Temple of Lilvani - it's located in the cave system that you explored during the Phantom of the Trade Route Witcher Contract.

After entering the chamber, a cutscene / dialogue will begin and you'll need to make a difficult decision...

Side with Gaunter O'Dimm

This is the simplest ending, by and large - just sit back and watch the interaction between Olgeird and Master Mirror unfold. Afterwards, you're allowed to choose a reward for aiding the mysterious merchant:

• "Wanna be as swift as the wind."
  Caparison of Lament Saddle. +100 Horse Stamina. Adds a black mist with a few flying insects around Roach's head, the description states that it "addles the mind of a randomly-selected opponent within a limited range." This puts a red and black glow around a random opponent's head and provides a few seconds worth of an Axii-type effect.
• "Never wanna go hungry again."
  Horn of Plenty. Provides an endless supply of food, offers +40 Vitality regeneration (25 in combat) for 5 seconds.
• "A bottle of vodka that's always full."
  Botomless Carafe. Provides an endless supply of alcohol to use in alchemic conconctions.
• "Wanna be rich."
  5000 Crowns
• "I don't want anything from you."
  Nothing. Except the sole ownership of your soul.

Side with Olgeird von Everec

If you choose to intervene and attempt to spare Ogleird's life, Geralt will challenge O'Dimm to a game of wits.

Once you awaken in the spirit realm, Master Mirror will pose a riddle to you:

To all things and men I appertain,
and yet by some am shunned and distained.
Fondle me and ogle me til you're insane,
but no blow can harm me, cause me pain.
Children delight in me, elders take fright.
Fair maids rejoice and spin.
Cry and I weep, yawn and I sleep.
Smile, and I shal grin.

What am I?

Once you set foot onto the pathway beyond the broken ruins, a timer will apear at the top of your screen. Instead of blithering about and inspecting every single item along the road, here's the path that will lead you not only to the answer, but to the best rewards (including new Witcher Gear and a sizeable pile of gold):

Follow the pathway straight, then fork to the right when you reach the open well with the two specrtes in front of it.

After crossing the wooden bridge and rounding the corner, keep heading straight on the path that leads under the large stone archway. Keep following this path (through the crooked trees) until you encounter a large spirit resembling a Leshen.

Use Spectre Oil on your blade to make quick work of the ghostly beast, then climb the broken stairs on the left of the path to pull the Venomous Viper Silver Sword out of the stone at the top. Turn back down the stairs and continue up the path on your left along the cliffside.

Climb up the rocks at the top of the path, then run past the large glowing tree and keep to the right until the path diverges. Follow the road ahead of you (the left fork)  until you reach the glowing cave entrance.

Along the entrance to the cave are small stacks of gold (5 crowns each). Pick those up if you want, or head directly into the cave and begin ransacking every crate, box and sack that you can. They're all filled with gold and precious gems, which can later be sold to merchants or craftsmen.

Upon exiting the money cave, head to your right and follow the path towards the large mansion. Enter the graveyard and run past the two Gargoyle spectres, instead running straight to the entrance of the house.

Head down the main hallway once inside to find the riddle's solution at the end. Of course, nothing can ever be too easy, so O'Dimm drops the floor out from under you, and you must find another way to solve the riddle.

Once you've dropped into the main courtyard, you can attempt to interact with any of the mirrors set up around the terrace - though they'll only self-destruct before you can use them.

Instead, head to the fountain opposite the door you entered from, and use Aard to destroy the wall behind it. This will allow a flow of water into the pool which you can use as a reflection to solve the riddle and complete the quest.