The scholar mage Calcelmo has racked up some strange book at the effigy of a wolf during his archaelogical digging. It seems to be about ancient, unique swords and, as he doesn't know what to do with it, he would be willing to sell it to you...
Once the book in your possession, you'll be able to conjure up a forge to craft your own Witcher swords. (To do so, open the book, choose "Forge - 1H" or "Forge - 2H", close the book and quit the menu; the forge will automatically open.)
They are swords made of steel or silver (with a bonus against the undead), some of them with a full scabbard and the others with a vanilla leather scabbard (WIP). You can forge a one-handed version and/or a two-handed version; they are craftable at two different forges accessible from the book. They can be crafted with ingots of steel/silver and corundum/other metals. You can temper them too. You'll need the Steel Smithing perk for all of them. Some of them are stronger than others and will require more advanced knowledge in the art of smithing.
Here is a list of the swords added in the game for now (from the less powerful to the most powerful):