Submitting my highest rated meme from Reddit, still the only class feature I know of that can work like this.

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 ability was designed for high level play. I would have tweaked it to deal 4d6 or 3d8, or just handwaved the massive damage rule.