Almost everything the party knows about Averis Thenn came from a single vision — the Vitriol Fulcrum’s memory, surfaced at the frozen gate of the Shardhold of Silence. What it showed was enough to reframe the entire mission.
He was not imprisoned. That is the first thing. Whatever the Shardhold is — fortress, prison, monument to something broken — Averis Thenn entered it voluntarily. The distinction matters. A prisoner is contained against their will. What Thenn did was something closer to a choice, which raises the question of what he was choosing between and what he understood about the cost.
He has been inside for centuries, holding something dormant. The Vitriol Fulcrum, for all her own complicated history with the Shardhold, has been the structure around that — the lock that kept whatever he is containing from finding its way out. Which means that any plan to destroy the Fulcrum, or to bring the Shardhold down, does not simply end her. It ends whatever arrangement Thenn made when he walked in.
Vladyn wants something tied to his soul. The most likely read is that Vladyn sees value in that connection — that binding some fragment of Thenn’s soul to himself would extend or deepen his hold over the seventh layer. Whether Vladyn understands what Thenn has been doing inside the Shardhold, or what would happen if that containment ended, is less clear. The party’s current suspicion is that he does not.
What Averis Thenn is, what he was before the Shardhold, and what the thing he has been holding actually is — none of that has been answered.