The Witcher 2: Assassins of Kings
Dancer is a steel sword in The Witcher 2: Assassins of Kings, added by the Enhanced Edition update. It can be found in the nekker cave while escorting Brigida through the forest during the Crown Witness quest. Along with Forgotten vran sword, Caerme, and Mourner, it is one of the best steel swords in the game. It is only accessible in chapter 3. (It is also possible to obtain a crafting diagram for Dancer in the Burned down hospital during the quest In the Claws of Madness in chapter I. Albeit extremely rare, it can be part of the loot in the chest behind the fake wall after the room where you fight the second pair of wraiths.)
The Witcher 3: Wild Hunt
Dancer returns as a steel sword in the third game.