Killing Xanathar

Not knowing what to do in this situation, you point your staff at the monster and wave it around a little. As you do, the top of the staff starts to glow like it’s about to do something, but it flickers out and nothing happens. As the beam hits you, your body starts to feel stiff and you can no longer move any of your body parts. Then a different eye-stalk turns pitch black and shoots a second beam at you. As it hits you, you don’t feel so good. You slowly start turning to dust and nothing is left except a pile of ash.