The recent discussion of what role Abaia plays in the new Urth prompts me to note the interest Abaia seems to show in the Claw.
Recall that in the beginning of CotC, Severian revisits the dream he had when he shared Baldanders's bed, and says: "Then very suddenly, I who had been blind before understood why it was that Abaia had sent me this dream, and had sought to enlist me in the great and final war of Urth."
Well thanks a lot Sev; care to enlighten the rest of us? Here's my guess:
Severian's recounting of the dream differs from his original description, inter alia, in the use of the words AZURE, SAND-GARDEN, and TALON. Anyone familiar with BotNS would immediately think of the Claw if given just those three words. Perhaps Abaia seeks to enlist Severian because, in addition to foreseeing that Severian will become Autarch, Abaia foresees that Severian will wield the Claw; Severian only realizes this after he learns that he possesses the Claw.
A connection is directly referenced when, at the end of CotA, Severian muses on where the Claw might draw its power from. He there speculates that the power of the Claw comes from Abaia & Co., who, having seen Severian's destiny, "found me again, and thereafter my course was twisted by the Claw."
In UotNS, the power of the Claw is shown to come from Sev himself. But Abaia's interest in the Claw remains. Idas, Abaia's minion, ransacks Severian's cabin looking for "something so small it might have been concealed in the collar of a shirt." While Severian is ready to torture Idas in order to get Idas to tell him what it was, I think it's a no-brainer for us that Idas was trying to find the thorn that will become the Claw - what else is there? Maybe then Idas is trying to prevent the Claw from coming into existence; though the thorn got a drop of Severian's blood when he first found it, I read Apheta's explanation ("It was saturated with your blood") to mean that the thorn did not become the Claw until Severian jammed it good into his finger. That takes place, of course, when Severian tries to use it to revive the steward. Whom Idas has killed trying to find the thorn. So Idas, in trying to prevent the Claw from coming into existence, is actually the reason it comes to be....