Regarding damage to an unconscious character the PHB says:
Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer
two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.
My question is do the characters accumulate damage that ISN'T at this hp max value. So if a hit at 30 damage would be just enough to kill the unconscious character , what about one hit at 15 in one round, and another hit at 15 in the next round? Does this kill them because it adds up to their max, or does it not kill them because it isn't the lethal hit that the rulebook seems to describe?
As a side question, if the characters do accumulate damage past 0hp, they are essentially going into negative hp. If this is how it works, does a healing potion have to heal that negative damage, or does any healing begin at 0hp and work upwards?
Best Answer
No.
The way damage works is that you take it until you get to 0 HP. On the strike that takes you to 0, you check the remaining damage against your max HP, if it's higher or equal to, you are dead.
If you suffer damage while at 0 HP, three things happen. First you suffer a death save failure. Second, if it's a crit, you suffer a second death saving throw failure (three failures means death). Finally if the damage you take meets or exceeds your max HP, you die.
However, the damage at 0 HP only counts for the single hit you are taking. Thus if you take 10 damage, and the next turn take 15, you are still at 0 HP, and just have two death save failures, but no further consequences.