Unconscious (Player’s Handbook, pg. 292)
• Attack rolls against the [unconscious] creature have advantage.
• Any attack that hits [an unconscious] creature is a critical hit if the attacker is within 5 feet of the creature.
Damage at 0 Hit Points (Player’s Handbook, pg. 197)
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.