RAW (Dropping to 0)

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.

For example, a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack, she is reduced to 0 hit points, but 12 damage remains. Because the remaining damage equals her hit point maximum, the cleric dies.

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious. This unconsciousness ends if you regain any hit points.

Unconscious

• An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.
• The creature drops whatever it's holding and falls prone.
• The creature automatically fails Strength and Dexterity saving throws.
• Attack rolls against the creature have advantage.
• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.