I think the verse is honestly pretty telling, combined with some other things points brought up in the game.
I've been mulling over a lot of this for a while but I haven't lurked here in a while so I don't know if anyone else has touched on anything similar:
We know that The Echo essentially works by touching the soul of someone else and that in Japanese it is referred to as the "超える力". 超える means "To exceed" but also "To cross over/pass through". And I think the Prime Ascian is incredibly representative of the latter, as by fusing into one Lahabrea and Igeyorhm essentially passed through the boundaries of each other existences and became one. The incident with the Sahagin was more or less the same thing. I think at it's very core, this is the true purpose of the Echo, allowing things to become one.
The Blessing of Light on the other hand I think is the direct opposite of that. Essentially, it's a wall. It's used to shield you from Ultima. What else do walls do? They separate and divide things along their boundaries. Which I think is ultimately the reason why the Prime Ascian is forced apart, based on the line after you defeat it right before they split back:
"What walls are these? Arghh, Damn you, Hydaelyn" or "魂の境界が造られている" lit. The boundaries of the soul have been constructed.
The verse plays right into that.
Before Hydaelyn and Zodiark separated, all creation was one. By separating Hydaelyn imposed boundaries upon creation, things were divided into 14 whatevers, and for the time being the balance was shifted far towards her. In response to this Zodiark created/recruited the Ascians to weaken her etc and cause the Rejoining. This would ultimately undo the boundaries created by Hydaelyn, everything would literally once again become one (i.e. form the Sea once again) and would quite literally be the end of the world as we know it, and the other terms for the Rejoining fit in with that.
Others have touched on the last two lines better than I can so I don't really have anything to add about those.
Now I'm rather fuzzy on this, but I believe we've been told that Echo users have been showing up in greater numbers more recently compared to the past. This I think is merely a symptom of Hydaelyn waning, and Zodiark growing stronger. People are simply being born with the "boundaries" of their soul weak enough that others can instinctively pass through just enough to see their memories etc. If people knew how to "push" harder on those boundaries it would likely result in another Prime Ascian type being coming into existence.