Gerun Oracles

[Nalien]

Agreed. It's only a one-off line of small talk, but Estinien clarly infers that Nidhogg sensed what was going on and abandoned the "torture them forever" plan and the "get back my eye" plan and decided to finally just raze the place to the ground.

I think it's a two-off actually. Once after we beat Vishap, and once more when we're preparing to travel with him to Dravania. Well, less of a change in plans for Nidhogg after Vishap, but still an indication that something was up;

But what of you, Nidhogg? Why did you not join your minions and make of this battle a slaughter? Have you grown so fond of the stench of your lair?
Mayhap you prefer to have your allies fight your battles for you... You have a girl to do your bidding now, do you not? No matter. I shall slay Iceheart, and all the rest...

Though I think prior to Steps of Faith Estinien made comment on Nidhogg ignoring him... "Please notice me, Nidhogg-Senpai!"

We now obviously know Iceheart wasn't even working with Nidhogg... So there has to be some other reason for him staying back. Come to think of it, that's a bit more evidence for Nidhogg really keeping an eye on Ishgard; Ysayle acted upon the whims of a trader from Ul'dah and Nidhogg jumped at the opportunity. He had no planning in that whole scheme, but his forces were still ready to take advantage of it. He was obviously very aware of current situations...

Perhaps a bit of a leap to go from that, to knowing Thordan was summoned, but it is still strongly implied that Nidhogg is still aware of his Eyes. Estinien knows what he is up to through it, and Nidhogg knows what Estinien is up to. Granted, we don't really have any idea where Haldrath was entombed, but it wouldn't be hard to believe that Nidhogg found out about Thordan that way. I mean Thordan VII did intend to fashion a sword out of the corpse...