The item is used to upgrade the consecrated sword and unlock the first skill. We'll tell you where to find it.
Everything is simple. Exit Fallgrim's tower through the door, enter the cave opposite and finish off Grisha. This is an optional boss, although it will be problematic to search the chest without destroying it. And, you guessed it, you will need to take the spike from the chest. Then return to the workbench in the tower and place the item on the consecrated sword. Your first skill is available!