Hi, I'm Treeston, and welcome to Combat PR Prep Refactors.

Today, we're moving UNIT_FLAG_IMMUNE_TO_PC and UNIT_FLAG_IMMUNE_TO_NPC to higher-level abstraction so combat manager can react to it.
New methods on Unit:
- void SetImmuneTo<All/PC/NPC>(apply, keepCombat = false);
- bool IsImmuneTo<All/PC/NPC>() const;

(cherry picked from commit 74af880217)

src/server/scripts/EasternKingdoms/ScarletMonastery/boss_headless_horseman.cpp

@@ -446,7 +446,7 @@ public:
                 headGUID.Clear();
             }
 
-            me->RemoveUnitFlag(UNIT_FLAG_IMMUNE_TO_PC);
+            me->SetImmuneToPC(false);
             //instance->SetBossState(DATA_HORSEMAN_EVENT, NOT_STARTED);
         }