So about the "warrior of darkness" [spoiler]

[treuhavik]

It was said (in the easter special event) that an Archon and also Echo user decided to side with "dark-hooded mages" for mysterious reasons.

Then there's Elidibus saying that "If we knew the truth we'd be of the same mind" that it has been a millennia since the last time he was around. I am guessing that he was able to recruit at least one echo user that was strong enough (or will be strong enough) to compete with WoL.

Also;

Scores of men and women have passed through our doors looking to find answers, and scores have embarked on quests to do just that...but their efforts have gained us little...and many have not returned.

Of those few who have...not one is willing to speak of what he saw at the end of the Path. It is as if they all bore witness to a truth too terrible...or too shameful to be told.

It's very likely that there are far more echo users than we know of that have <ahem> gone to the dark side after witnessing this "truth", so I'd label this much of your speculation plausible.
(Aside: "Archons" used to be echo wielders as well, of whom I assume were far fewer in number than they are today. I believe that this latest Ardor, or attempted Rejoining (7th Calamity), was, in fact, special in some way as to cause Hydaelyn to awaken so many.)

EDIT: I've been Moose'd!

[Plegethorn]

I've actually been thinking on this topic recently, and I have a theory, if you guys are willing :3

So, drawing inspiration from older FF games, I'll begin.

In the older games (mainly 3) Warriors of Darkness are called when the Light in the world becomes too strong and threatens to destabilise it. They are usually chosen by the crystal to bring a balance. Sometimes working with the WoL, sometimes not.
It is my thought that Eorzea is becoming unstable due to the amount of darkness that the WoL has actually banished. So with that in mind, the mothercrystal felt need to interfere, lest you destabilize the world.

Another idea focuses on FF4.

In FF4, there existed Crystals of Light and crystals of Darkness. What if the Ascians actually hold the Crystals of Darkness? They could plead with them for a Warrior of darkness, possibly by claiming that light was threatening to destabilize the realm. Said Warrior would then start doing things to introduce Darkness back into the world.

[Kaiser-Ace]

I've actually been thinking on this topic recently, and I have a theory, if you guys are willing :3

So, drawing inspiration from older FF games, I'll begin.

In the older games (mainly 3) Warriors of Darkness are called when the Light in the world becomes too strong and threatens to destabilise it. They are usually chosen by the crystal to bring a balance. Sometimes working with the WoL, sometimes not.
It is my thought that Eorzea is becoming unstable due to the amount of darkness that the WoL has actually banished. So with that in mind, the mothercrystal felt need to interfere, lest you destabilize the world.

Another idea focuses on FF4.

In FF4, there existed Crystals of Light and crystals of Darkness. What if the Ascians actually hold the Crystals of Darkness? They could plead with them for a Warrior of darkness, possibly by claiming that light was threatening to destabilize the realm. Said Warrior would then start doing things to introduce Darkness back into the world.

Now I've not played FF3 personally but I recall the world of Darkness having it's own 4 elemental crystals that were independent of the world of Light's Crystals. It may be that the Warrior of Darkness is Zodiark's Champion but Hydaelyn almost certainly has nothing to do with him as he's the Ascians (Seemingly Eledibus' in specific) Trump Card as Eledibus him self seems to have resorted to the Warrior of Darkness as a last ditch counter measure.

On that note though, the only "Balance" we as the WoL are threatening is the Status Quo. We're taking Control away from the Ascians and gaining so much power that now they have to be afraid of us and plan in relation to our actions instead of ahead of them. Being the control freaks that they are (Especially our white robed friend) I'd wager they aren't very happy with the status quo shifting against them since they want to tip the scale towards darkness not maintain balance. (We've solidified balance now the scale threatens to tip in favor of light)

[lokoxDZz]

I've been reading the topic but i kinda feel why people not mentioned the Crystal of Darkness? At some point of the story( i forgot which i think its before nabriales fight) we have a clear vision of a Crystal of Darkness just like Hydelyn. It could be just as we are the chose of Hydaleyn the WoD is chosen from the Crystal of Darkness

[Kaiser-Ace]

I've been reading the topic but i kinda feel why people not mentioned the Crystal of Darkness? At some point of the story( i forgot which i think its before nabriales fight) we have a clear vision of a Crystal of Darkness just like Hydelyn. It could be just as we are the chose of Hydaleyn the WoD is chosen from the Crystal of Darkness

Presumably that Crystal of Darkness was Zodiark, The Ascian God.

[Toranja]

I've been reading the topic but i kinda feel why people not mentioned the Crystal of Darkness? At some point of the story( i forgot which i think its before nabriales fight) we have a clear vision of a Crystal of Darkness just like Hydelyn. It could be just as we are the chose of Hydaleyn the WoD is chosen from the Crystal of Darkness

That thing is not awoken yet. That's what the whole Ascian scheming is about.

[Akiza]

Speaken of Zordiark being asleep I don't think Hydaelyn is fully awake yet either since she is still in crystal form. In Final Fantasy when God's are awake they take on a humanoid form.

[myahele]

Scores of men and women have passed through our doors looking to find answers, and scores have embarked on quests to do just that...but their efforts have gained us little...and many have not returned.

Of those few who have...not one is willing to speak of what he saw at the end of the Path. It is as if they all bore witness to a truth too terrible...or too shameful to be told.

Makes me wonder if echo users were recruited by the Asciens, especially by Lahabrea? Elidibus seems to have been away for a 1000 years, but we know he's tried to reason with and/or recruit Minfilia and the WoL; now he's got the WoD.

Additionally, Lahabrea and Elidibus were also "of this (Hydaelyn) world" and

The SMN questline Lahabrea's lackey asciens said they were "elevated" by Lahabrea

[treuhavik]

Makes me wonder if echo users were recruited by the Asciens

Well, who else of this world is more likely to know of this "truth too terrible" than the ones who so eagerly tease us about the truth of our power...?