Yennefer was born with a physical deformity that resulted in a hunched back and dislocated jaw. Growing up in the city of Vengenberg, she was constantly rejected and insulted by the townspeople, and hated by her own father due to her deformity. While being attacked by two of the townspeople, her magic manifested and she instinctively teleported herself away. She arrived in Aretuza, the magical academy for sorceresses. She was sent back to her town by a fellow mage.

Shortly after, a woman named Tissaia arrived to Yennifer's home and purchased her from her father for a measly four coins. After teleporting into Aretuza, Yennifer was magically marked, which allowed Tissaia to track her down and bring her back to the academy to study magic. Having been sold by her own father and forcefully taken away from her home, as well as from a childhood full of trauma and neglect, Yennefer became suicidal and attempted to slit her own wrists. She was saved by Tissaia, and began her magical training.

Although the beginning of her magical training was filled with failures and disappointment, Yennefer eventually turned out to be one of the most powerful and accomplished mages at Aretuza and was chosen to "Ascend". She underwent a magical surgery/transformation that fixed her deformities but left her infertile. After years of training and decades of experience as a sorceress out in the world, she met Geralt of Rivia, whom she developed feelings for...

For what came next, and for a more detailed history of Yennefer, check out her Wiki page.

Yennefer doesn't have many feats in terms of physicals/skill, but the few she has are still pretty good.

Yennefer is still conscious after being struck by a lightning bolt and sent flying. Another girl was KO'd after being struck by the lightning. Yen also absorbed some of the bolt's energy.

Without any anaesthesia, Yen undergoes a painful transformation involving surgery, alchemy, and magic, in order to fix her physical deformations. The first step of this procedure involves removing her reproductive organs. The second step involves creating a paste with the organs, applying it to her skin, and then having her spine and jaw violently straighten and correct themselves. Every sorceress that undergoes this procedure is given herbs to reduce the pain, but Yen refuses them and endures the full pain, showing she has immense pain tolerance: https://youtu.be/nEIpNEPSge8?t=7 (***NSFW***)

Yennefer uses a sword and a dagger to fight off and kill several warriors, alongside Geralt. These warriors are all trained, armed, and larger than her, but they are no match for Yen's superior sword-fighting skills.

Yen even manages to save Geralt and kill his attacker after he got sand tossed into his eyes.

Gets stabbed in the stomach by an arrow.

And then fights off a second attack by the mind-controlled Sabrina before getting blown off a multi-story tower, and lands on her feet. Despite the stab wound, Yen continues to fight in the battle.

Creates a portal to escape a mage assassin and its pet.

After arriving in a desert, creates a second portal to escape the assassin and its pet.

Within the span of two seconds, creates a portal and disappears through it. Since the portal appeared at Yen's location, it's likely she created the portal on herself.

Given the vast differences in environment that Yen creates portals to within the span of a few minutes (snowy forest, sunny desert, rainy town, mountain, field), she can travel at least a few dozens of miles away.

Precise enough to control a creature's appendage to slice its own head off.

Unleashes a telekinetic blast against the assassin, who protects himself with some sort of black smoke shield. Yennefer then creates a portal to escape but gets sliced through the shoulder with a thrown knife as she disappears.

Moves a mirror so that Geralt can't see her enter the bathtub: https://media.giphy.com/media/gIl4cigqQVeAoGecAO/giphy.gif (***NSFW***)

Pushes Jaskier against a wardrobe: https://media.giphy.com/media/h7jArXG273Z7VsQaDn/giphy.gif (***NSFW***)

Tosses Geralt a few feet away with enough force to crack the wall: https://media.giphy.com/media/Wmd8K6ZlGH6kAro5Y2/giphy.gif (***NSFW***)

Yen wakes up just in time to stop a huge fireball and then redirect it away. You can see in the second GIF how large the fireball's explosion is.

Kills two soldiers in quick succession with internal attacks. Note how this was right after Yen was stabbed in the stomach and tossed off a tower from an explosion.

Throughout most of the Battle of Sodden Hill in the last episode, Yennefer is in the watchtower, using her telepathy to direct the other mages' attacks and monitor the enemy soldiers from hundreds of meters away. This involves reading the minds of the approaching soldiers and telepathically communicating with the other mages to be ready & attack.

The first GIF involves Yen telling Vilgefortz to slow down and reserve his Chaos. She even calls out his name in concern, showing that she can see how the battle is going.

The second GIF involves Yen monitoring the approach of the enemy soldiers until they are in range of the defences' arrows. She is doing this via telepathy since she couldn't see into the forest.

Uses the word "Ragamuffin" to end the mind control spell she had on at least two dozen people, making them have sex for her own amusement. In the same scene, she was able to detect Geralt's slower heartbeat and deduce that he was resistant to her mind control.

Puts Geralt to sleep with a kiss and then makes him go on a rampage to get arrested and imprisoned. This is impressive given Geralt's willpower and how Witchers are stated to be immune or at least highly resistant to mind spells such as Axii.

Missing Feats:

• As a novice, reads Istredd's mind well enough to see, hear, and taste what he is imagining.
• Created an illusion of an audience while having sex with Istredd.
• Blocks her deepest thoughts from Istredd.

In a moment of stress, Yen instinctively teleports herself away and ends up in Aretuza. This was the first demonstration of her magical abilities.

Yen casts a slowing spell on the assassin's creature as it is mid-leap; she then creates a portal to escape.

Yen attempts a healing spell on a baby, but it fails because the baby is already dead.

Having been struck by a lightning bolt rather than catch it in a bottle like Sabrina successfully did, Yen unleashes a lightning bolt against her in a fit of jealousy, but it is caught and redirected by Tissaia.

Casts a spell using some incense that cures a man's erectile dysfunction and induces an erection.

Yen put Jaskier in a healing sleep in order to cure the Djinn-afflicted magical curse that was on him and would have killed him.

Yennefer enchants her tent to make the inside much larger and spacious than what it looks like from the outside: https://youtu.be/LMUnD3MCIMw?t=10

Casts an immobilization spell on a group of four dwarves, which stays in effect for several minutes. Even as the spell starts to wear off, the dwarves' movements are sluggish for several seconds.

Casts another immobilization spell on an enemy warrior while in combat.

Yen absorbs a large amount of fire in order to fuel her power.

She then unleashes her power against the enemy army by releasing a massive stream of fire. This attack devastates the army, killing over 30 000 soldiers and burning down a large portion of the forest, while also completely ignoring Tissaia and the surviving mages. It was stated by Tissaia that this act is what turned the tide of the battle and allowed their side to win. Note that Yen was still suffering from a knife wound to the stomach and had been fighting all day long.

The first episode of Season 2 explains that Yennefer passed out and was teleported away and taken prisoner by the enemy army. However, instead of dying from having depleted her Chaos, Yennefer lost her magic instead.

At the end of S2, after recovering her magic, Yennefer heals Coën's wound:

Missing Feats:

• Takes a feather in her hands and casts a spell to multiply it into 6 feathers.

Since all of the mages underwent the same training at their respective magical academies, it is likely that Yen could replicate these same spells, especially since she's considered one of the most powerful mages. Furthermore, Vilgefortz claims that Tissaia stated to him that Yennefer was the best student she ever taught.

Also, Yennefer should be able to replicate any magic sign that Geralt has used, since witchers use basic spells in and out of combat.

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Stregobor enchants the inside of a tower with an illusion of a lush garden filled with naked women. This illusion is good enough to fool Geralt's enhanced senses: https://www.youtube.com/watch?v=EhtD3Sb_pSs (***NSFW***)

Tissaia transmutes three of the novice mages into eels.

Triss causes four soldiers to fall asleep.

Triss creates a wall of branches to block a doorway, manipulating it to disarm and incapacitate the attacking soldiers.

A random mage uses a continuous wind blast to hold back a group of soldiers, and then creates a telekinetic shockwave to knock away another group of soldiers.

A sorceress uses all her Chaos and withers away after creating a large fireball.

Vilgefortz portals in and kills several soldiers. After tossing his sword, he casts a spell to conjure up another one in his sheathe.

From a distance away, Triss conjures up mushrooms from the ground that cause the soldiers to die.

Coral uses an internal attack to simultaneously kill a group of roughly two dozen soldiers.

Fringilla conjures up a box of silver parasitic worms that burrow into people's ears and puts them under her control.

Fringilla casts a spell that breaks open the wooden doors of the keep, from hundreds of meters away and without line of sight.

Fringilla conjures a ground portal into the keep and has archers fire arrows through it.

Mousesack creates a magical barrier at the castle's doors. It repels three soldiers charging into it and he maintains it for several hours while volleys of fire arrows are shot at the barrier. One arrow somehow manages to get through, and once the door starts burning, he drops the barrier.

By touching a dead soldier, Tissaia is able to receive a vision of the last moments leading up to his death.

Tissaia touches Cahir and causes him excruciating pain for several seconds.

Tissaia touches Cahir's head and telepathically probes his thoughts and memories, which causes immense pain for Cahir since he is attempting to resist her.

Sabrina heats up a pool.

Triss heals Ciri's wound within a few seconds.

Stregobor creates an illusion for the Brotherhood. Note how the fiery illusion emits light.

Stregobor creates an illusion to make it look like his hands have been removed.

Stregobor touches Yennefer and puts her in a mental illusion, where he then interrogates her. He then tears through Yennefer's memories, causing her pain. Tissaia dispels the illusion and then sends Stregobor flying several meters away into a wall.

Rience, a mercenary mage, puts up a shield of fire in order to protect himself from Vesemir's Aard sign. Triss casts a spell and blocks Rience's fireball with a blast of wind. Rience then traps Triss in a circle of fire. Rience burns Vesemir's stick and KO's him with a fireball. Triss extinguishes the fire trap, and Rience portals away.

Rience traps Ciri within a circle of fire. Geralt attacks Rience with Aard (which he blocks with a fire shield), and the sign also extinguishes the circle of fire.

Rience channels a stream of fire in order to burn through a metal door, although it takes him a few minutes to do so, and he portals away before Geralt can get him.

Istredd puts up a magical barrier to protect himself from Geralt, who Triss sent via portal from very far away.

Istredd knocks a soldier off his horse with a magical blast.

Francesca puts curses on the doors of several houses, which burns sigils onto the babies within. Francesca completes the spell with a gesture and all of the babies die: https://www.youtube.com/watch?v=1DYZVS2dNh0

Statements:

• Mousesack offered to make Ciri fall asleep.
• Fringilla accidentally froze a cat.
• Sabrina turned her mother fat.
• Fringilla, who was in Yennefer's class at Aretuza, created a storm that sunk 50 ships. This was done off-screen. Although there are no details for this (i.e. how long it took, how it worked, etc.), it's still an impressive feat. Both Yennefer and Fringilla agreed that the former was more powerful, meaning Yennefer could replicate this as well.

Missing Feats:

• One of Fringilla's mages covers the forest in fog but then withers once he arrives at the doors.
• A random mage created a shockwave that knocked back about seven soldiers.

• Fringilla conjures up a small animal for her, Yennefer, and a few others to eat. She comments that Yennefer could conjure up a boar for them to eat if she wasn't bound in magical chains.
• Francesca, an elven mage introduced in Season 2, is able to make Fringilla pass out with a gesture.
• Triss casts a spell to enter the deepest layers of Ciri's consciousness, in order to help her piece together her history.