void main()
{
    int nDoOnce = GetLocalInt(OBJECT_SELF, "DO_ONLY_ONCE");
    if(nDoOnce == 1)
        return;
    SetLocalInt(OBJECT_SELF, "DO_ONLY_ONCE", 1);

    object oPC = GetEnteringObject();
    object oDoor = GetObjectByTag("frost_cave");

ApplyEffectToObject(DURATION_TYPE_PERMANENT, EffectVisualEffect(VFX_DUR_GLOW_PURPLE), oDoor);
}