[Script/ICC] Properly fix Dark Nucleus aggro problems and them not following you when they should.

Closes #6589
Fix by @Marosa. Guidelines by @Kandera.

src/server/scripts/Northrend/IcecrownCitadel/boss_blood_prince_council.cpp

@@ -1296,7 +1296,7 @@ class npc_dark_nucleus : public CreatureScript
 
             void Reset()
             {
-                me->SetReactState(REACT_PASSIVE);
+                me->SetReactState(REACT_DEFENSIVE);
                 me->CastSpell(me, SPELL_SHADOW_RESONANCE_AURA, true);
             }
 
@@ -1315,15 +1315,12 @@ class npc_dark_nucleus : public CreatureScript
 
             void MoveInLineOfSight(Unit* who)
             {
-                if (me->GetDistance(who) >= 15.0f)
-                    return;
-
                 ScriptedAI::MoveInLineOfSight(who);
             }
 
             void DamageTaken(Unit* attacker, uint32& /*damage*/)
             {
-                if (attacker == me || attacker == me->getVictim())
+                if (attacker == me)
                     return;
 
                 me->DeleteThreatList();
@@ -1345,6 +1342,8 @@ class npc_dark_nucleus : public CreatureScript
                             DoCast(victim, SPELL_SHADOW_RESONANCE_RESIST);
                             me->ClearUnitState(UNIT_STATE_CASTING);
                         }
+                        else
+                            MoveInLineOfSight(me->getVictim());
                 }
                 else
                     _targetAuraCheck -= diff;