Killing Xanathar

Not knowing what to do, you’re body seems to move on its own and unsheathes your sword and try’s to block the beam. Right before the blade makes contact with the beam, it starts to glow the same bright color of red that you saw with the crystal when it was glowing. When the beam hits the sword, it shoots back at Xanathar. Right as hits Xanathar, you see it stop to move and it stays there unmoving for a few seconds.