Sorry for the double post, but if we're going there, I suppose I'll further the digression from the thread topic and share my latest musings on the subject.
*equips his own darksteel-foil hat*
What I believe to be the defining characteristic of Eikons is that they are naturally occurring. What I mean by this is that an Eikon is, in essence, more primordial than the primals we know. They are the 'originals', born of the land and returning to it upon death, much like a primal, but are able to return at will instead of relying on someone to summon them, as they are actual beings of the world's aether. The only way to stop an Eikon from appearing would be to lock it away and keep it from death.
It's hard not to draw similarities between the means of their here assumed existence/survival and the immortality the Echo provides those who've mastered it. Also, being of the planet, it only makes sense to envision them as guardians of it.
What do you do when you're trying to alter the world and the life forms of it and are confronted by aetherial protectors of the that same planet? Well, I suppose you'd dispose of themin one way or another. And if death isn't an option, why not capture them and use them as fuel for your aetherochemical research?
Conversely, what do you do when the protectors you depended upon suddenly and seemingly abandon you? You pray for their return, probably near their preferred environment -- a place likely sharing in that being's elemental aspect.
And a primal is born...
Unlike an Eikon, a primal is a thoughtform given life by the desire for their appearance and enough aether to physically manifest it. To drive this point home, even if a primal is sealed, there's nothing yet stating that another copy of it could not be subsequently summoned. An example of this would be Bahamut. Even though a primal already going by the name Bahamut was kept in a spherical prison orbiting the planet, another Bahamut could've been summoned if Tiamat had felt like giving it another shot.
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When we first saw Midgardsormr 'awaken' in The Keeper of the Lake, I couldn't help but assume that he, too, possessed the power of the Echo, seeing as his 'soul' was still present and that he was able to commune with Hydaelyn.
This brings to mind another question, a question of whether or not Hydaelyn, the deity that we know, shares more than just a name with the planet Midgardsormr, and ourselves, protect.
Merging what little we know of Midgardsormr's arrival on Hydaelyn, he came bearing the seeds of Dragonkind and this occurred nigh-simultaneously with the 'creation' of water...
Going back to Eikons, we know of four: 'Goddess', 'Fiend', 'Demon' and Odin; each bearing a different appearance... Did each of them birth their own race...?
If so, Midgardsormr is a perfect fit for our fifth-discovered Eikon, and it's easy enough to pin him as attributed to the element of water.
And IF Midgardsormr truly is a 'primordial' could he have been mistaken as a primal by an outside nation, halting their invasion in the same year he destroyed an entire fleet of their finest...?
That's all for now. Consider the pot stirred. Loremonger Vicious out.