Try a derivation of this:
IF
CharacterReceivedDamage(CHARACTERGUID_DC_VoidCat_3b96fa75-22e1-4410-bde5-0d469e18fae4)
AND
CharacterGetHitpointsPercentage(CHARACTERGUID_DC_VoidCat_3b96fa75-22e1-4410-bde5-0d469e18fae4,_Percentage)
AND
_Percentage <= 50
AND
GlobalGetFlag("VoidCatEnrageMode",0)
THEN