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) { }