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
Was about to post something similar.
IF
CharacterVitalityChanged(CHARACTERGUID_Grand_Father_Dick_31f20125-f047-4a8a-8b86-b9b2b9a67264,_)
AND
CharacterGetHitpointsPercentage(CHARACTERGUID_Grand_Father_Dick_31f20125-f047-4a8a-8b86-b9b2b9a67264,_Hp)
AND
_Hp < 90
THEN
DisplayText(CHARACTERGUID_Grand_Father_Dick_31f20125-f047-4a8a-8b86-b9b2b9a67264, "Less than 90% health");