Gaunter O'Dimm, also known as Master Mirror or the Man of Glass, is a malevolent, enigmatic entity of immeasurable power who wanders the world granting just about any wish to certain individuals in need, but at a price as the wish backfires on them and he ultimately comes to claim the price of their soul. He is very likely the most powerful character in the entire franchise by far. Gaunter O'Dimm creates a storm the second time he meets Geralt, wreaking the ship he was captured in.
Gaunter O'Dimm can see the true nature of things, such as when Vlodimir possesses Geralt.
Gaunter O'Dimm can freeze and manipulate time.
Gaunter O'Dimm is aware of events and history, such as knowing about Geralt and Shani's history together.
To be claimed by Gaunter O'Dimm (And hear Gaunter O'Dimm's true name) is a fate worse than death.
https://witcher.fandom.com/wiki/Gaunter_O'Dimm The Witcher 3: Hearts of Stone said:
Gaunter O'Dimm: All who have learned my true name are now either dead or have met an even worse fate.
Gaunter O'Dimm is a visitant (refered to as being demonic, but also implied to be an elemental) who was summoned from the outer spheres by Olgierd to bestow immortality on him and his wife Iris.
https://witcher.fandom.com/wiki/Blood_Pacts,_or_On_Foreign_Presences Blood Pacts said:
Whosoever summons a Visitant into our Realm risks much. Whether flickering as a Spirit in the ether or inhabiting the Flesh of a designated Host, the Visitant shall strive to attain its Liberty, and if it does, it shall wreak great havoc. Thus a Visitant must be summoned only within a protective Circle, which shall bind it as no fetters could.
The Circle must be complete and eternal, with no ruptures. Its line must be drawn with a steady hand firmly gripping the chalk. Since a stray step, made through carelessness or the Visitant's trickery, can smudge the line, the Circle must be further secured by a barrier in the form of lit candles made of beeswax. A Circle thus protected shall stand as a sure and inescapable Prison for the Visitant.
Between Worlds said:
Of all the beings inhabiting the Outer Spheres, the spirits of the elementals are the most foreign to human nature. They possess a will of their own and are intelligent, though not in the way of men. For there is no way to fathom their drives and intentions.
Genies, the most powerful of the elemental spirits, prove the most difficult for men to commune with. Only the true masters of magic have perfected this art, which demands long years of study and risky experiments. Yet even the most adept at the magic arts cannot tame a genie, merely force it to perform certain acts against its own volition.
They say the mage Stammelford once commanded a d'ao to move a mountain blocking the view from his tower, but even he could never convince a genie to be a willing helpmate in his work or to give sound advice, for the reasoning of these creatures is beyond human understanding.
Professor Premethine Shakeslock, although blinded from the knowledge of knowing about Gaunter O'Dimm, could see Gaunter O'Dimm clearly when he came to visit. Gaunter O'Dimm draws a circle on the ground to protect Shakeslock from him, and warns him that he'd die if he leaves it. When Shakeslock is forced to leave the circle, he slips on a bottle and breaks his neck, being killed instantly. Shakeslock says that Gaunter O'Dimm is evil itself.
Demons who exist in multiple dimensions simultaneously (as I will get to below) are afraid of Gaunter O'Dimm.
Should you choose to save Olgierd, Gaunter O'Dimm takes you to another plane of existence that he rules over and is seemingly one with, and possibly created. Within this world he decides the rules.
Said dimension has a moon and many stars within it.
When banished, Gaunter O'Dimm calls Geralt insignificant, and says he will return (in three different languages).
The Witcher 3: Hearts of Stone said:
Gaunter O'Dimm: You are insignificant. You think you've defeated me but you are wrong. I can't be killed, I will be back.
Gaunter O'Dimm was the one to place a curse on Marlene.
https://www.eurogamer.net/articles/2016-10-23-the-making-of-the-witcher-3s-greatest-villain The use of a spoon led to it becoming a symbol of Master Mirror's influence, or a calling card. CD Projekt decided to include the object in the Olfieri prison ship earlier on. CD Projekt then used this symbol of power to remind players of his reach in the next expansion, Blood and Wine, where Geralt meets a cursed wight that's obsessed with collecting spoons. It's made pretty clear who placed this curse upon the unfortunate creature.
"It felt natural to have the Master Mirror's curse manifest itself in some way that involved spoons, especially if that curse regarded food," Stachyra explains. "Though Gaunter O'Dimm was not one of the main characters of [Blood and Wine's] story, we really wanted to show that his presence was not just limited to the encounters with Geralt and Olgierd von Everec.
"So, together with Paweł Sasko, the designer behind the wight spoon collector quest, we wanted to suggest that Master Mirror had a hand in Marlene's story. No one mentions his name, but the fact that it was indeed the Man of Glass who cursed Marlene is pretty clear from the story she tells Geralt. And when you add spoons to that, it becomes obvious that Gaunter O'Dimm continues to travel the world and test what we call 'human nature'."