Libram of Forbidden Arts is a relic piece obtained from Evil Javid, the 2nd Boss in the Mage Tower. Alternatively, you can purchase it from the Adventure Shop for 400 Smith Tokens any time you want.
The Libram of Forbidden Arts is the core component in the Relic Fusion to create the Artifact Sword, Forbidden Sword of Holy Incantations(75 AP, 15 DP & Arcane Element).
The other pieces you need to create Forbidden Sword of Holy Incantations are the 3 Rank-A Bronze Swords, Conqueror, Manafury and Demon Stone Infused Blade. As you can see from the below picture, it will tell you when you have the correct swords. The below picture only has 1 correct sword.
Successfully fusing the 4 components together over 36 hours will result in your sword.