Dawn may banish even the darkest night - Obviously transition from Umbral era into Astral.
Yet ever shall primal desires burn. - A nod towads Bahamut, if not all of the Primals in general.
Two swords shall vie to lay them low--
A blade born of light and a blade forged of might.
Alas, man may entrust his fate unto but one.
My theory on the last few verses...
Ishgard and Garlean.
Ishgard is known as being something of a church ruled archetype (right word? not sure actually). Where, in this game's version, is ruled by their own Pope like figure.
That's my theory on 'Blade born of Light'
and the Garlean Empire, being 'blade forged by might'
Ishgard is more inclined towards the divine, holy influences. Whereas Garlean is against magic, but rather focuses on machineary and industry to work their means into ends. Thus the 'forging' part of the verse.
So, might it be concluded that at some point through the storyline we'll have an option to either procure Garlean technology, either by stealing it and having Cid build us something for our own use. Or something like a temporary alliance with the Garleans if we should do such a thing to lay down Bahamut for mutual need then return to fighting against them.
As one particular old phrase goes "The enemy of my enemy is my friend" albiet tentively.