King of mists weird death by totem after killing him (campaign)

My friend killed the king of mists in the campaign and after killing him he died. He shared his screen and I saw a totem pulsing every 1-2 seconds with a huge purple AOE. He said he died in two pulses. Anyone knows what this is? I’ve killed the boss more times than I can count but it’s the first time I see something like that.

Exoskeleton78
1 | 9 hours ago