This is a joke… one-shotted by unknown killer… was farming easy content… was on full life, buffed, fortified, more than enough armor and res, monsters could not hurt me at all and suddenly dead… do not play HC !!!
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I got one tapped with 600 000 toughness when doing the world boss…
Yes, happend just right now to me too 
It is a frustating thing that happend and happend since beginning of Diablo IV but as Blizzard don’t adress such dead in HC we have to live with it!
It’s quit a shame, but small company 
Hi, it happened to me too. I was farming easy content in Penitent difficulty with paladin, level 53, and suddenly died. I thought it was weird, and I went to the Hall of the Fallen, and there was the killed by unknown killer message. Has anyone from the dev started to have a look at this problem? Dying in HC for my errors is fine and part of the game, but dying randomly without knowing why is not.
Unknown killer is probably a death from an environment effect like an explosion or poison, something the system couldn’t relate to a certain creature.
I read your level 53 when you died. Those last 10 levels from 50 to 60 you have to be careful. The gear jumps from roughly 500 to 750. So your very quickly undergeared.
In first playthroughs I mostly stay in normal or hard till 60. The reason is low life. Once you get the best temper for max life, then it becomes a lot easier.