Since Thordan VII notes that both of Nidhogg's eyes have been kept in the Holy See's safekeeping, the only place Haldrath could have been entombed would be Ishgard itself... which, given Nidhogg's ability to sense his eyes and things near them, lets a rough timetable for the background for Heavensward be hashed out.

1. Ascians give Thordan VII the power to become King Thordan.
2. Nidhogg notices thanks to his Eye and sets his sights on Ishgard with the intent to raze it because he knows now they can win.
3. Steps of Faith.
4. Nidhogg regroups and our journey begins.

Basically, Thordan VII used us to kill Nidhogg so Ishgard wouldn't be razed and he could use his other Eye, the one with Haldrath, to power up relatively unmolested.

Clever old geezer... but, what does this have to do with the Gerun Oracles?