Instant Death

Some spells and abilities can kill you immediately or bring you closer to death without needing to reduce you to 0 Hit Points first. These abilities have the death trait and usually involve energy from the Void, the antithesis of life, or other effects that target the mind or soul. If you're reduced to 0 Hit Points by a death effect, you're slain instantly without needing to reach dying 4. For instance, the quantum negation spell automatically kills a creature it reduces to 0 Hit Points.
A death effect might state that it kills you outright, without dealing damage, in which case you die without having to first reach dying 4 or be reduced to 0 Hit Points. For instance, the massacre spell, an extremely powerful spell, can automatically slay creatures who fail their save against its effects.

You die instantly if you ever take damage equal to or greater than double your maximum Hit Points in one blow.