[Spell] Net Guard 11920 should reduce threat on target hit

Might be different by entry that uses the parent spell

Confirmed for:
c.16912 145200 -> 72600
c.1034 19400 -> 9700
c.485 seemed to work different

Based on vmangos/core@619b0d7

Closes #714

sql/scriptdev2/spell.sql

@@ -39,6 +39,7 @@ INSERT INTO spell_scripts(Id, ScriptName) VALUES
 (26569,'spell_hate_to_zero'),
 (26637,'spell_hate_to_zero'),
 (10255,'spell_stoned'),
+(11920,'spell_net_guard'),
 (12639,'spell_summon_hakkar'),
 (12948,'spell_hakkar_summoned'),
 (13278,'spell_gdr_channel'),

src/game/AI/ScriptDevAI/scripts/world/spell_scripts.cpp

@@ -863,6 +863,18 @@ struct RetaliationCreature : public SpellScript
     }
 };
 
+// 11920 - Net Guard
+struct NetGuard : public SpellScript
+{
+    void OnEffectExecute(Spell* spell, SpellEffectIndex effIdx) const override
+    {
+        if (effIdx != EFFECT_INDEX_0)
+            return;
+
+        spell->GetCaster()->getThreatManager().modifyThreatPercent(spell->GetUnitTarget(), -50);
+    }
+};
+
 struct HateToHalf : public SpellScript
 {
     void OnEffectExecute(Spell* spell, SpellEffectIndex effIdx) const override
@@ -1217,6 +1229,7 @@ void AddSC_spell_scripts()
     RegisterSpellScript<PreventSpellIfSameAuraOnCaster>("spell_prevent_spell_if_same_aura_on_caster");
     RegisterSpellScript<InstillLordValthalaksSpirit>("spell_instill_lord_valthalaks_spirit");
     RegisterSpellScript<RetaliationCreature>("spell_retaliation_creature");
+    RegisterSpellScript<NetGuard>("spell_net_guard");
     RegisterSpellScript<HateToHalf>("spell_hate_to_half");
     RegisterSpellScript<HateToZero>("spell_hate_to_zero");
     RegisterSpellScript<Stoned>("spell_stoned");