In The Book of the New Sun, there is a mausoleum in the necropolis of Nessus by the Citadel. The young Severian often goes there to contemplate when he's in the Guild of Torturers.
There is a device graved in bronze above the door, which Severian says he has adopted as his own. The door itself was sprung "long ago", and remains "eternally ajar" since it's too heavy to move. However, in The Urth of the New Sun,
it's closed by the water masses when the Citadel is flooded.
There are five coffins in the mausoleum, two empty ones on the floor and three intact on a shelf along one wall. There is a funeral bronze there depicting a man:
The dead man lay at full length, his heavy-lidded eyes closed. In the light that pierced the little window I examined his face and meditated on my own as I saw it in the polished metal. My straight nose, deep-set eyes, and sunken cheeks were much like his, and I longed to know if he too had dark hair.
I suppose at this point in the book, knowing what little we do then about Severian's past before becoming a torturer at young age, we might suppose the dead man is Severian's father. (Interestingly enough, Severian mentions that on some of his visits to the hidden mausoleum, he rests/sleeps in one of the empty coffins. And at a later point, Severian calls the funeral bronze "my funeral bronze"… ) After reading all the books, though, including The Urth of the New Sun,
we know that later on the older Severian travels back in time several times, sometimes even leaving behind corpses (Apu-Punchau). Is this one of those? Is it the Conciliator, one of Severian's time travelling incarnations?
This connection seems confirmed in The Citadel of the Autarch:
I know now the identity of the man called the Head of Day, and why Hildegrin, who was too near, perished when we met, and why the witches fled.
(This, of course, referring to the aforementioned Apu-Punchau.) And then, immediately after, so definitely connected:
I know too in whose mausoleum I tarried as a child, that little building of stone with its rose, its fountain, and its flying ship all graven. I have disturbed my own tomb, and now I go to lie in it.
There are also passages here that make it seem like
The man in the mausoleum is Severian, but from a different timeline somehow. This doesn't mesh with the understanding of the time travel we see in The Urth of the New Sun, but still, that's what I interpret it as; that the Severian in the tomb is a different, alternate Severian who never found the Claw of the Conciliator, and that someone (who?) travelled back in time to change something that occured in his youth, turning him into "our" Severian who found the Claw. But then, why do these two Severians exist in the same timeline?
The mausoleum also seems connected to the rebel Vodalus somehow, although that might just be me reading too much into it. The aforementioned bronze-graven device consists of a fountain rising above waters, a rising ship, and a rose. When Severian receives a coin from Vodalus, the obverse of the coin portrays Vodalus's face. Then he notices this:
At last I turned my treasure over, and then indeed I caught my breath; stamped on the reverse was just such a flying ship as I had seen in the arms above the door of my secret mausoleum. It seemed beyond explanation – so much so that at the time I did not even trouble to speculate about it, so sure was I that any speculation would be fruitless.
He then goes back to the mausoleum and buries the coin there to hide it. For some reason, when Severian later returns to the mausoleum and looks at the coin, it no longer portrays Vodalus on the obverse…