Killing Xanathar

Not knowing what else to do in this scenario, you point the staff at the monster and wave it around a little hoping it will do something, if anything. As you do, the top of the staff starts to glow and a blue sphere appears around you. Once the beam makes contact with the orb, it suddenly gets shot back at the beast in front of you. Xanathar then stops moving dead in it’s tracks and falls to the ground.