Hmm, in rereading The False Sun, I noticed something interesting:
Together they pulled down Nogaral, the High Round, raised it into a heap over the mouth of the Well.
[...]
At last they paused to regard their labour, the Inchoroi alighting upon the same spectral floor that bore Shaeönanra. Crimson sunlight bathed the southern ramps, inking the numberless crevices across the wrack and ruin. And they rejoiced, Man and Inchoroi…
They had no inkling of the greater violence their sorcery had unleashed.
The sky cracked. Iros shuddered. The impossible sun tipped and stumbled. Plumes of ejecta exploded from points along the mountain’s perimeter, scarcely visible for the Diurnal’s encompassing glare. The mound that had been Nogaral shrugged then slumped into its contradiction. It was as if a dome of cloth had been pressed into a dimple. Summit became basin. Illumination became shadow. The mountain had been rotten with Viri, its innumerable ways fractured by the cataclysmic impact of the Ark thousands of years before. The underworld mansion imploded, collapsed inward and downward, tier upon tier, hall upon hall, undone by this final indignity. This last outrage.
It seems that something exploded in The Well?
Crackpot incoming...
What if it wasn't a nuke at all that Kellhus found, but the nominal False Sun, buried when the Nogaral was destroyed, and what if KELLHUS made it overload and explode, destroying the horde and killing Saubon, while simultaneously telling his Ordeal to flee to save as much as possible. Surely Kellhus had heard of the betrayal of Titurga and the False Sun he carried.
Probably not the case, but an interesting thought. It wasn't Tekne at all.