Instant Death

Here are the main ways a creature can die instantly.
Hit Point Maximum of 0. A creature dies if its Hit Point maximum reaches 0. Certain effects drain life energy, reducing a creature's Hit Point maximum.
Massive Damage. When damage reduces a character to 0 Hit Points and damage remains, the character dies if the remainder equals or exceeds their Hit Point maximum. For example, if your character has a Hit Point maximum of 12, currently has 6 Hit Points, and takes 18 damage, the character drops to 0 Hit Points, but 12 damage remains. The character then dies, since 12 equals their Hit Point maximum.