Tempering

[Enkidoh]

I think it's an interesting concept.

That a Primal can relinquish their claim on a tempered soul, effectively curing them of tempering. However, because a Primal needs to voluntarily give up a worshipper, which fuels their power, I can't see Ifrit, Garuda and the like doing so.

I don't know how it is with Elder Primals though...

Ironically, this is actually what version 1.0 seemed to be hinting at when you first encountered Ifrit during the Path of the Twelve main scenario quest 'Lord Errant' - in the cell in Zan'rak just prior to being taken to the Bowl of Embers, the player encounters two npcs, a lalafell girl in a robe and coif curled up in a foetal position (named 'Cloudy-Eyed Prisoner') and an elezen male in a doublet reclining back and named 'Emotionless Prisoner'. Talking to the two revealed they were already tempered (the elezen specifically mentions "being tempered in the flames" like he was), and when the player and their Path Companion are then taken to the Bowl of Embers, those two prisoners are included with them, and then apparently tempered again.

This seemed to suggest that tempering was an imperfect process that had to be continuously performed in order to keep tempered under a Primal's thrall, but all the information shown in ARR seemed to retcon this, with it mentioned time and time again that tempering was permanent and that a tempered was pretty much "better dead" than to be allowed to live. If the info revealed during the Second Coil is correct, this would actually ring true with the events with Ifrit during 'Lord Errant' in 1.0.

Considering how Odin is, they definitely seems to be summoned differently then the others.

Maybe they are like some permanent natural forces that just can't be erased.

All Primals are stated as being immortal beings that cannot be truly destroyed - they're literally just essences that float through the river of aether (I dislike using the FFVII-reference-term 'Lifestream', as it's a retcon - 1.0 only ever called it 'the river of aether through the planet'), and that the summoning process effectively creates a physical form for them to inhabit, powered by the aether in the crystals they consume as part of the summoning rite and the devotion given to them by their followers (read: tempered).

This explains why they were sealed in Silvertear Lake - the Allag knew they couldn't be truly destroyed and so the next best thing was simply to seal them away to prevent them from being summoned, Odin and Bahamut obviously being so powerful as to require their own individual sealing (Odin in Urth's Font and Bahamut within Dalamud).