The Witchers’ Forge is a side quest in Kaer Morhen in which you’ll search for an old forge that contains some powerful weapons and armor.
You can start this quest at any time, but you will not be able to complete it before you get the Eye of Nehaleni from Keira Metz, during the “Wandering in the Dark” main quest.
Following contains quest spoilers. You’ve been warned
Read a copy of the tome.
From the main hall at Kaer Morhen, head to the southeastern tower, with the massive stone pillar in its center, and look for a chest in the middle of the three arched alcoves. In it you’ll find a book – read it immediately. There is a note on the margin in the book, something about the breaking of an illusion for which the book is the key.
Go to the mine near Kaer Morhen.
Head south from Kaer Morhen, to the Old Mine signpost on the western bank of the river. From it follow the path west until you reach the entrance to the mine itself.
Search the mine.
Go down the tunnel until you reach the first chamber of the mine. There you’ll run into an earth elemental guarding a stone pedestal. Kill it, then inspect the pedestal. Place on it the book you’ve previously found; a section of the cavern’s wall will collapse. Go through it and into the next chamber.
Search the mysterious chamber.
You’ll see a forge brimming with magic. Climb up to the raised area with the fireplace and use the Eye of Nehaleni to spot an illusion. Interact with the bellows there and the fire will speak to you and warn you that the forge belongs to Radmir of Tor Carnedd. Choose whichever conversation option you wish, but tell it that you want to look around. The fire will not be happy at that and will rise up and blast you across the chamber.
Kill the Ifrit.
You’ll now have to slay the ifrit. You should first check the bestiary for information on how to achieve that. The best tactic is to cast Aard while the ifrit is ablaze, and to strike it with heavy attacks while dodging its own attempts to hit you.
Remove the illusion.
When you kill the ifrit, approach the hearth and use the Eye of Nehaleni to see a secret compartment behind a section of a wall that is an illusion. There you’ll find three chests with some armor and weapons. The quest has now come to a close.