The events surrounding The Plan fall solidly into the Weavers' reign.
What would become the Deepnest faction left during their initial struggle against GMS, probably from Weavenest Atla. (Hornet's faction silkspear, use of runes, & it's implied some of them looked at her as a second chance of what Eva was supposed to be - this faction wasn't sucessful at "wresting their freedom" & eventually fled. )
At some point the faction of Weavenest Askar (the one on on Mt. Fay) suceeds & gets the drop on GMS. They crafted the bells (prototypes are found there), & they used clawline, & a clawline-using Weaver is buried beneath the citadel. (presumably died while in charge of it)
Herrah's faction goes to Deepnest, lives there a while, establishes themselves as part of its community, Herrah marries their King, etc. eventually they make contact with Hallownest which goes poorly at first - but eventually they ally against the Radiance & begin extensive trade, Hornet is born, the infection returns, the Weavers flee.
They return to Pharloom, where their kin have since come out on top, bringing their knowledge with them.
The Weaver in Greymoor might've been someone from the return faction - she has a similar design to the ones from Deepnest (no spikes) & can do threadstorm, one of the techniques Hornet already knew in the 1st game. Note that some of the citadel enemies (I think it was the grand reeds?) can do an attack kinda like threadstorm (which Hornet comments on as resembling/ likely having been taught to them by the Weavers) - they might also have brought with them knowledge of the void, leading their compatriots to investigate it & build Weavenest Absolom.
However, they're unable to find permanent solutions to GMS or fix their reproduction problems, so they die out, hoping that maybe some of the other escapees will return with the means to that.
At this point the Conductor's take over (Over in Hallownest, the second infection outbreak would've left the place depopulated)
The Conductor era was long (the citadel started out as a stone building & endedgolden & gaudy; They grew more & more obssessed with silk switching greymoor from farming to silk collecting, at some point they started to use robots... Second Sentinel was one of the most advanced robots, too, so they wouldn't have been among the first. )
They would've got deactivated around the time the haunting took over the citadel (a few generations before silksong's present) - this is also around when Lace & Phantom might've been created, or, realistically, a little bit before the actual takeover.
Which is to say: Second Sentinel could be pretty old, but not as old as any of the Pale King's offspring.
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