Used in the monk 2.0 epic click 
Peace of the Disciple and everybody's favorite MPG trial 
Test of Efficiency, also used in some player spells like 
Wake of Atrophy.
code...
spell effects
	Code:
	Index: spell_effects.cpp
===================================================================
--- spell_effects.cpp	(revision 1602)
+++ spell_effects.cpp	(working copy)
@@ -2812,6 +2812,7 @@
 			case SE_LimitCastTime:
 			case SE_NoCombatSkills:
 			case SE_TriggerOnCast:
+			case SE_MaxHPChange:
 			{
 				break;
 			}
 mob.h
	Code:
	Index: mob.h
===================================================================
--- mob.h	(revision 1602)
+++ mob.h	(working copy)
@@ -582,7 +582,7 @@
 
 	inline sint32	GetHP()			const { return cur_hp; }
 	inline sint32	GetMaxHP()		const { return max_hp; }
-	virtual inline sint32	CalcMaxHP()		{ return max_hp = (base_hp  + itembonuses.HP + spellbonuses.HP); }
+	virtual sint32	CalcMaxHP();
 	float GetWalkspeed() const { return(_GetMovementSpeed(-47)); }
 	float GetRunspeed() const { return(_GetMovementSpeed(0)); }
 	float GetBaseRunspeed() const { return runspeed; }
 mob.cpp
	Code:
	Index: mob.cpp
===================================================================
--- mob.cpp	(revision 1602)
+++ mob.cpp	(working copy)
@@ -596,6 +596,25 @@
 	return max_mana;
 }
 
+sint32 Mob::CalcMaxHP() 
+{
+
+	max_hp = (base_hp  + itembonuses.HP + spellbonuses.HP);
+	
+	int slot = GetBuffSlotFromType(SE_MaxHPChange);
+	if(slot >= 0)
+	{
+		for(int i = 0; i < EFFECT_COUNT; i++)
+		{
+			if (spells[buffs[slot].spellid].effectid[i] == SE_MaxHPChange)
+			{
+				max_hp += max_hp * spells[buffs[slot].spellid].base[i] / 10000;
+			}
+		}
+	}
+	return max_hp;
+}
+
 char Mob::GetCasterClass() const {
 	switch(class_)
 	{
 client mods.cpp
	Code:
	Index: client_mods.cpp
===================================================================
--- client_mods.cpp	(revision 1602)
+++ client_mods.cpp	(working copy)
@@ -240,6 +240,18 @@
 
 	max_hp += GroupLeadershipAAHealthEnhancement();
 	
+	int slot = GetBuffSlotFromType(SE_MaxHPChange);
+	if(slot >= 0)
+	{
+		for(int i = 0; i < EFFECT_COUNT; i++)
+		{
+			if (spells[buffs[slot].spellid].effectid[i] == SE_MaxHPChange)
+			{
+				max_hp += max_hp * spells[buffs[slot].spellid].base[i] / 10000;
+			}
+		}
+	}
+	
 	if (cur_hp > max_hp)
 		cur_hp = max_hp;
 	return max_hp;