Asphyxiate

Reaching out into the area around you, you create a void around a creature's head, making it impossible for them to breathe.

When you cast this spell, a creature you can see within range must make a CON saving throw. On a failure this creature begins to suffocate, and can only hold it's breath for a number of rounds equal to it's CON modifier. After this time has elapsed, the creature must make a 15 CON saving throw at the start of it's turn, or fall unconscious. If the creature succeeds, it must make another saving throw at the start of it's turns for the spell's duration, the DC  increases by 5 every turn. If a creature succeeds a saving throw twice, the spell ends.