The rules on "Instant Death" state:
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum […]
Does this only apply if you aren't already at 0 HP? Or rather, what happens if you are currently at 0 HP and take massive damage?
Best Answer
Yes, Massive Damage can kill you at 0 HP
Mathematically, this is equivalent to the normal rule that occurs when a character is above 0 hit points, so it makes perfect sense that they would reimplement the rule here in precisely this manner.