Since that's a L17 player ability, I'm betting the point is that reducing to 0hp is generally more damage than (55) 10d10. So as a DM I'd rule it that it still takes you to 0 and doesn't do massive damage (instant kill)
        this post was submitted on 15 Jul 2023
        
      
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    They're introducing the BBEG to a level 1 party, so that barbarian probably only has around 10 hp
Don't classes start at max HP? So he has atleast 12, plus whatever con modifier he got going.
The specific details aren't important, but yeah. A lot less HP than can survive 10d10 damage.
That IS true, of course.
This ability was designed for high level play. I would have tweaked it to deal 4d6 or 3d8, or just handwaved the massive damage rule.