Core/AI: CreatureAI.EnterCombat is now called CreatureAI.JustEngagedWith. There's also two new methods on UnitAI, though they're never invoked right now.

Port From (https://github.com/TrinityCore/TrinityCore/commit/bce43de7f3fba618c370f2aba327d23dd30d1cdd)

Source/Game/AI/CoreAI/CombatAI.cs

@@ -51,7 +51,7 @@ namespace Game.AI
             }
         }
 
-        public override void EnterCombat(Unit victim)
+        public override void JustEngagedWith(Unit victim)
         {
             foreach (var id in Spells)
             {
@@ -134,7 +134,7 @@ namespace Game.AI
             AttackStartCaster(victim, _attackDist);
         }
 
-        public override void EnterCombat(Unit victim)
+        public override void JustEngagedWith(Unit victim)
         {
             if (Spells.Empty())
                 return;

Source/Game/AI/CoreAI/CreatureAI.cs

@@ -408,11 +408,8 @@ namespace Game.AI
             me.DoImmediateBoundaryCheck();
         }
 
-        /// <summary>
-        /// Called for reaction when initially engaged
-        /// </summary>
-        /// <param name="victim"></param>
-        public virtual void EnterCombat(Unit victim) { }
+        // Called for reaction when initially engaged - this will always happen _after_ JustEnteredCombat
+        public virtual void JustEngagedWith(Unit who) { }
 
         // Called when the creature is killed
         public virtual void JustDied(Unit killer) { }

Source/Game/AI/CoreAI/UnitAI.cs

@@ -478,6 +478,13 @@ namespace Game.AI
         public virtual void SetGUID(ObjectGuid guid, int id = 0) { }
         public virtual ObjectGuid GetGUID(int id = 0) { return ObjectGuid.Empty; }
 
+        // Called when the unit enters combat
+        // (NOTE: Creature engage logic should NOT be here, but in JustEngagedWith, which happens once threat is established!)
+        public virtual void JustEnteredCombat(Unit who) { }
+
+        // Called when the unit leaves combat
+        public virtual void JustExitedCombat() { }
+
         public virtual void DamageDealt(Unit victim, ref uint damage, DamageEffectType damageType) { }
         public virtual void DamageTaken(Unit attacker, ref uint damage) { }
         public virtual void HealReceived(Unit by, uint addhealth) { }

Source/Game/AI/ScriptedAI/ScriptedAI.cs

@@ -48,7 +48,7 @@ namespace Game.AI
                 DoStartNoMovement(target);
         }
 
-        // Called before EnterCombat even before the creature is in combat.
+        // Called before JustEngagedWith even before the creature is in combat.
         public override void AttackStart(Unit target)
         {
             if (IsCombatMovementAllowed())
@@ -411,7 +411,7 @@ namespace Game.AI
         public override void Reset() { }
 
         //Called at creature aggro either by MoveInLOS or Attack Start
-        public override void EnterCombat(Unit victim) { }
+        public override void JustEngagedWith(Unit victim) { }
 
         public bool HealthBelowPct(int pct) { return me.HealthBelowPct(pct); }
         public bool HealthAbovePct(int pct) { return me.HealthAbovePct(pct); }
@@ -502,7 +502,7 @@ namespace Game.AI
                 instance.SetBossState(_bossId, EncounterState.Done);
         }
 
-        public void _EnterCombat()
+        public void _JustEngagedWith()
         {
             if (instance != null)
             {
@@ -602,7 +602,7 @@ namespace Game.AI
         public virtual void ScheduleTasks() { }
 
         public override void Reset() { _Reset(); }
-        public override void EnterCombat(Unit who) { _EnterCombat(); }
+        public override void JustEngagedWith(Unit who) { _JustEngagedWith(); }
         public override void JustDied(Unit killer) { _JustDied(); }
         public override void JustReachedHome() { _JustReachedHome(); }
 
@@ -635,7 +635,7 @@ namespace Game.AI
             summons.DespawnAll();
         }
 
-        void _EnterCombat()
+        void _JustEngagedWith()
         {
             Unit target = SelectTarget(SelectAggroTarget.Random, 0, 0.0f, true);
             if (target)
@@ -684,7 +684,7 @@ namespace Game.AI
 
         public override void Reset() { _Reset(); }
 
-        public override void EnterCombat(Unit who) { _EnterCombat(); }
+        public override void JustEngagedWith(Unit who) { _JustEngagedWith(); }
 
         public override void JustDied(Unit killer) { _JustDied(); }
     }

Source/Game/AI/SmartScripts/SmartAI.cs

@@ -609,7 +609,7 @@ namespace Game.AI
                 me.GetMotionMaster().MoveIdle(); // wait the order of leader
         }
 
-        public override void EnterCombat(Unit victim)
+        public override void JustEngagedWith(Unit victim)
         {
             if (IsAIControlled())
                 me.InterruptNonMeleeSpells(false); // must be before ProcessEvents

Source/Game/Entities/Unit/Unit.Combat.cs

@@ -1443,7 +1443,7 @@ namespace Game.Entities
                 if (enemy != null)
                 {
                     if (IsAIEnabled)
-                        creature.GetAI().EnterCombat(enemy);
+                        creature.GetAI().JustEngagedWith(enemy);
 
                     if (creature.GetFormation() != null)
                         creature.GetFormation().MemberEngagingTarget(creature, enemy);