Strangle
Prerequisite: 5th level and must have Grappler Diverse Ability
Effect: When attempting to grapple a creature, if you succeed by 5 or more on your Strength (Athletics) check, you can choose to strangle the creature. Your speed is reduced to 0, attacks against you have advantage, and the target begins suffocating.
Suffocating
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.
For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.