Thanedd coup was an attack between mages during one of their conclaves on Thanedd Island in July of 1267. Originally meant to be one of their normal meetings, the mages were secretly split between supporting Nilfgaard and the North and both planned to use the meeting to catch the other side unawares. In the end, violence broke out and many were killed or injured and the Brotherhood of Sorcerers was effectively dissolved.
Emperor Emhyr, having suffered a humiliating loss in the last war during the Battle of Sodden Hill thanks to the North's mages, resolved to take care of the mages before another attempt at war with the North. For this, he employed Vilgefortz, a member of the Chapter, to help stage a coup and take out the opposing mages in the Brotherhood. Vilgefortz soon learned that things weren't as harmonious between the Northern rulers and their mage advisors either: instead of employing their mages to send messages to each other, the rulers were using the longer and older method of royal messengers to ride to and fro, having grown wary of their mages growing too politically powerful. Using this, Vilgefortz leaked this information to the mages who then called for a conclave at Thanedd Island to discuss how to deal with the new situation.
Meanwhile, the northern rulers planned to stage an attack at Dol Angra to make it look like Nilfgaard had attacked Lyria and Aedirn, giving them an excuse to attack Nilfgaard and take Cintra back. However, Emhyr came to know of these plans and sent a large battalion just across the border, waiting for the right excuse. King Vizimir II of Redania and his head of intelligence, Sigismund Dijkstra, learned of this betrayal and sent word to Aedirn's king, Demavend, not to stage the attack as now Nilfgaard was prepared to attack if they went through with it. However, the royal messenger was killed while traveling to Aedirn and Demavend never received this crucial information.
Mages began to arrive at the magical island for the meeting and held their usual banquet beforehand, where everyone mingled and acted cordially. However, there were heavy undertones about everyone's political alliance and warning each other who a mage spied for, even if they were technically on the same side. Several non-mages were also included in these festivities: Geralt of Rivia, who'd been brought along by Yennefer, and Dijkstra, who'd been brought along by Philippa Eilhart.
The next day the two groups of leaders, the Chapter and the Council, were to meet in Garstang, a tower on the island that had been embedded with spells to prevent any magic inside it, making it safe for heated discussions between mages. Unbeknownst to the Northern supporters, Francesca Findabair, another member of the Chapter and Nilfgaard supporter, had successfully snuck in a boat of Scoia'tael beneath the hollowed island and remained hidden, waiting to enter through Garstang and take out the North's supporters during the next day's meeting there. Despite not knowing this, the Northern supporters knew there were traitors among them and made plans of their own and instead of going to bed, they stayed awake through the night to make their move while the rest went to sleep.
Just before dawn, the Northern supporters made their move and began to arrest the Nilfgaard supporters with the help of Dijkstra and his Redanian soldiers, handcuffing the traitors in Dimeritium shackles before taking them to Garstang to be tried and hanged for treason. While they were trying to prevent as much violence as possible, Lydia van Bredevoort, Vilgefortz's assistant, was killed in the commotion, though her death was actually caused by Vilgefortz himself to buy him and Rience time to prepare. Despite this, all the Nilfgaard supporters were successfully arrested without much trouble and taken to Garstang, where all the mage advisors to kings were also called even if they remained neutral, like Dorregaray.
Once in the tower, Philippa began to draw up the charges against the group, including Vilgefortz, Francesca, Artaud Terranova, and Fercart. When the arch-mistress of Aretuza, Tissaia de Vries, interrupted, she pointed out that all the Northern supporters were also biased, backing kings rather than being independent mages, and that she knew of the plans the North had for war but to make it look like Nilfgaard started it. When Philippa pointed out that the Nilfgaard supporters weren't any less innocent, having decided to back the Empire after being promised to rule over the Northern lands if they won, Tissaia demanded proof. However, as proof could be fabricated, the arch-mistress instead brought in Yennefer with Ciri, who had been put in a trance to act as a medium, as one couldn't lie while in an augury. In her trance, Ciri revealed the truth behind the attack in Dol Angra where Demavend tried to use it to incite war against Nilfgaard. However, as it'd been a technical attack against the Empire, Emhyr's troops answered back and began to push into Aedirn and Lyria. She then further revealed that King Vizimir II had been assassinated last night.
As Philippa cried out this was all a lie, Tissaia made up her mind and remove the magical spells preventing the use of magic in the tower and freed the Nilfgaard supporters from their shackles and a battle broke out immediately, with several dying within the first few minutes and the Scoia'tael being allowed in by the now freed Francesca, causing more chaos. Seeing the bodies, Tissaia tried to calm down Vilgefortz to stop the killing, but the sorcerer merely laughed in her face and continued. Distraught, Tissaia ran off while the fighting continued between the two sides. Neither side ended up truly winning, as many had died or been injured during it.
Following the coup, confidence in mages seriously declined and many of them were expelled from the royal courts. It is also cited as one of the precipitating events of the war that followed. For her role in the coup, Francesca Findabair was made queen of Dol Blathanna, a free elven state, under the condition that she would openly denounce the violent actions of the Scoia'tael, while allowing them to continue to harass the Northern Kingdoms. With the fall of the main leaders of the Brotherhood of Sorcerers, the organization was dissolved and a few weeks later Philippa and other sorceresses forming the Lodge of Sorceresses, but this time also included Nilfgaardian mages.
- In The Witcher 2: Assassins of Kings, a book called The Thanedd Coup can be purchased in Vergen and provides a journal entry about this event.