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Chapter 8: Playing the Game / Encounter Mode / Special Battles / Aquatic Combat

Drowning and Suffocating

Source Player Core pg. 437
You can hold your breath for a number of rounds equal to 5 + your Constitution modifier. Reduce your remaining air by 1 round at the end of each of your turns, or by 2 if you attacked or cast any spells that turn. You also lose 1 round worth of air each time you are critically hit or critically fail a save against a damaging effect. If you speak (including Casting a Spell) you lose all remaining air.

When you run out of air, you fall unconscious and start suffocating. You can't recover from being unconscious and must attempt a DC 20 Fortitude save at the end of each of your turns. On a failure, you take 1d10 damage, and on a critical failure, you die. On each check after the first, the DC increases by 5 and the damage by 1d10; these increases are cumulative. Once your access to air is restored, you stop suffocating and are no longer unconscious (unless you're at 0 Hit Points).