Is it even possible to forcibly summon a deity as it is with Primals? I may be wrong here but I was always under the impression that the 12 let their presence be known on their "own terms" if you will. As far as we know Primals can manifest when there is a high concentration of aether and/or through summoning rituals. It would seem odd to me that Rhalgr, a deity could be summoned by similar means. If this is not the case, why didn't the Ishgardians call upon Halone?