My "Pit Of Death" particle effect despawns just a second after despawning

Hi, So i'm working on my first boss and basically what i want him to do is when he reaches 80% hp he's supposed to spawn a "Pit Of Death" which is just 3 block wide area containing the black reddust particle and whenever someone enters the pit they're supposed to take 3 hearts of damage per second. The pit shouldn't despawn until the boss is dead.

Atm i dont know how to make it so that it spawns just once when he reaches 80% hp so i have it set to spawn whenever he gets hit until i figure...

My "Pit Of Death" particle effect despawns just a second after despawning