Holy crap! <Late Main Quest Spoilers>

[Tenkuu]

First of all, I apologize in advance for the necro-posting, but this is a really interesting thread.

So, going by that, and what we see in the ending, we know this Crystal of Darkness was in the form of a necklace around Thancred’s neck. We see it shatter inside of the Echo where the player meets with Hydaelyn. The question is: when exactly did that necklace binding appear? We’ll analyze the timeline.

Since I'm right in the middle of these quests right now on an alt, I can tell you that immediately after taking the quest to go see the Sylphs from Minfilia, you can walk outside of the Solar and talk to Thancred who will just be standing there, and he will already have the dark crystal necklace. I don't actually know whether it's a bug that he has it this early given the fact that in the cutscene where you get the Sylph quest, he's there and not wearing it, but either way, that's the earliest that I've spotted him wearing it.

So we know from the quest “Escape from Castrum Centri”, that it was Alphinaud who suggested Thancred investigate the Ascians. When exactly then, did he take off for the mission? Right when the player heads to investigate the Sylphs and a possible threat of Ramuh summoning.

When you're reporting the Sylph quests to Minfilia, everyone is there in the office except for Thancred who, both before and after you report it, is sitting near the entrance to Waking Sands on a stool, again wearing the necklace. Before you actually walk in during that cutscene though, Alphinaud is there talking with Minfilia, and among other things they discuss an unnamed matter which they are having Thancred investigate, and the way Alphinaud is talking it seems like Thancred just gave him an update. There's almost no doubt that this is the Ascian investigation.

If the player returns to the Waking Sands, Thancred is nowhere to be found. Y’shtola, and Urianger are in the storage room, as are several other Scions. Yda and Papalymo, of course, are assisting the player with the quests in the Shroud.

I don't know about if you return during the Sylph quests, but as mentioned above, right after you get the Sylph quests, you can already find him wearing the necklace, whether this is a mistake or not.

But this also brings up the question: if my theory is wrong, and if the possession took place immediately, why is it then, that Thancred would willingly suggest Y’shtola accompany the player to Titan’s lair, instead of summoning Livia and the horde of Garleans to raid the place, knowing that the player is there? The purpose of Livia’s raid was to capture the player, after all.

I think Thancred was himself when he said that, but assuming for a moment that he wasn't, it could simply be Lahabrea playing a game of divide and conquer, especially since he probably would have known that Yda had also remained in Gridania. For me, this reinforces the notion that Lahabrea only meant to separate us from the Scions, and perhaps even carefully kept the strongest combatants such as Thancred, Yda and Y'shtola away so that the raid would have no chance of failing.

I think your theory is correct, but my personal opinion has always been that Lahabrea's and the Garlean's goals were different. I think Lahabrea was more interested in separating us from the other Scions whereas the Garleans wanted to capture us to study the Echo. Just look at us between the raid and Alphinaud's appearance at the church: we don't know what to do. I think Lahabrea knew we would lose our purpose without the guidance of the Scions.

And if he was still himself when he suggested Y’shtola go with the player to Titan, how is it that he does NOT know about the crystal around his very neck? Did he willingly make a pact with the Ascians? Some kind of plea bargain, or perhaps, he tried to take on something he could not handle on his own? Speculate away!

The necklace is fairly inconspicuous, to be fair, but given what Thancred says after the 2.0 endgame, he was likely well aware of having it and didn't think it posed a danger to him, and thus fell into Lahabrea's trap. The way he says it, he took on more than he could handle without realizing it.