A character who is dropped to =/< 0 HP with a single hit is knocked unconscious, and must pass a death saving throw or be healed within their next 3 turns.
If they do not pass the death saving throw or are not healed within that time, the character is dead. If they are successfully hit by an enemy before getting healed or passing a death saving throw, the character is dead. If a critical hit (nat 20) from an enemy drops a character to a negative value of half HP (ex: XP of 20 dropped to -10 or lower) the character is dead with no option for healing or death saving throw.
If they pass a death saving throw, their HP is set at 1 and they gain 3 points of exhaustion until a short rest is completed. If successfully healed, they gain 1/2 of the rolled HP and gain 2 points of exhaustion until a mini rest is completed.