A_JumpIfHealthLower
From ZDoom Wiki
A_JumpIfHealthLower (int health, int offset)
A_JumpIfHealthLower (int health, str "state")
Jumps the specified amount of states forward if the health of the calling actor is lower than the given value.
Examples
This script would cause a boss monster to switch its mode of attack when it reaches 5000 health
States {
Missile:
POSS E 2 A_FaceTarget
POSS E 2 A_JumpIfHealthLower(5000, "LowHealthAttack")
POSS F 2 A_PosAttack
LowHealthAttack:
POSS E 2 A_CPosAttack
}