Stops the default behavior of the spell and uses the spell's data to make a permanent spawn. Tested and works with http://www.wowhead.com/spell=62743/undercity-banner and similar spells.

AddToSpell.sql

REPLACE INTO `spell_script_names` (`spell_id`, `ScriptName`) VALUES (62743, 'spell_save_object');

SpellSpawn.cpp

#include "ScriptPCH.h"

class spell_save_object : public SpellScriptLoader
{
public:
    spell_save_object() : SpellScriptLoader("spell_save_object") { }

    class spell_save_object_spellscript : public SpellScript
    {
        PrepareSpellScript(spell_save_object_spellscript);

    private:
        static bool SpawnObject(Player* player, uint32 objectId, const WorldLocation* summonPos)
        {
            const GameObjectTemplate* objectInfo = sObjectMgr->GetGameObjectTemplate(objectId);
            if (!objectInfo)
                return false;
            if (objectInfo->displayId && !sGameObjectDisplayInfoStore.LookupEntry(objectInfo->displayId))
                return false;
            float x, y, z, o;
            summonPos->GetPosition(x, y, z, o);
            o = player->GetOrientation();
            Map* map = player->GetMap();

            GameObject* object = new GameObject;
            uint32 guidLow = sObjectMgr->GenerateLowGuid(HIGHGUID_GAMEOBJECT);

            if (!object->Create(guidLow, objectInfo->entry, map, player->GetPhaseMaskForSpawn(), x, y, z, o, 0.0f, 0.0f, 0.0f, 0.0f, 0, GO_STATE_READY))
            {
                delete object;
                return false;
            }

            // fill the gameobject data and save to the db
            object->SaveToDB(map->GetId(), (1 << map->GetSpawnMode()), player->GetPhaseMaskForSpawn());

            // this will generate a new guid if the object is in an instance
            if (!object->LoadGameObjectFromDB(guidLow, map))
            {
                delete object;
                return false;
            }

            sObjectMgr->AddGameobjectToGrid(guidLow, sObjectMgr->GetGOData(guidLow));
            return true;
        }

        void SaveObject(SpellEffIndex effIndex)
        {
            PreventHitDefaultEffect(effIndex);
            const WorldLocation* summonPos = GetExplTargetDest();
            Player* player = GetCaster()->ToPlayer();
            if (!summonPos || !player)
                return;
            SpawnObject(player, GetSpellInfo()->Effects[effIndex].MiscValue, summonPos);
        }

        void Register() override
        {
            // OnCast += SpellCastFn(spell_save_object_spellscript::SaveObject);
            OnEffectHit += SpellEffectFn(spell_save_object_spellscript::SaveObject, EFFECT_0, SPELL_EFFECT_TRANS_DOOR);
        }

        uint32 _targetCount;
    };

    SpellScript* GetSpellScript() const override
    {
        return new spell_save_object_spellscript();
    }
};

void AddSC_spell_save_object()
{
    new spell_save_object();
}