Claw of the Conciliator: Chapters 10 to 12
Severian and Jonas eat a special meal with Vodalus, which involves taking a drug from an alien creature, the alzebo, and consuming the flesh of Thecla. Jonas manages to hide the fact he does not partake, but Severian, due to the drug and ritual, absorbs Thecla's memories. And thanks to his peculiar memory, they're not going to fade.
Vodalus gives Severian the job of delivering a message to a servant in House Absolute, and Severian and Jonas head north on destriers provided. They are attacked by a flying shadow creature, the notule; Severian tricks the creature to attack and kill a nearby soldier instead.
(*) I laughed out loud at the reveal that the destrier that Severian stole to go to the cave belonged to Vodalus.
(*) It's revealed in passing that one of the many things Severian is trying to accomplish is find Dorcas again. Something he mentioned to Jonas. So that's something.