And another thing!
Kiernan gave her a strange look. “You can speak freely with me, my lady. I already know that it holds the process for purifying and draining beast cores of power without shattering them, allowing us to turn them into pure celerium that can be used as Conduits.”
She did this when she swallowed a beast core and the nightmare claimed credit. Obviously she couldn’t swallow a beast core the size of a pomelo - Myrddin’s conduit was that size - so if the nightmare is telling the truth perhaps he did trap some kind of sentience in Carnagore and used that to convert beast cores into celerium?
Or the nightmare was lying and it’s something to do with her particular bloodline? Or a trait specific to nulls? To be able to extract the magic and…metabolise it in some form? Presumably Myrddin would have found a way to copy this process without actually getting some poor sod to make a living swallowing and excreting rocks.
Edit: third option - the Sebastian body is specifically formulated for this kind of transformation. Look, I’m throwing every kind of batshit theory at the comments at this point. Is it Thursday yet?
Yes, this whole thread is becoming a review of all the pertinent information in books past. But how can I speculate at the contents of the future books without dissecting the information from the past books to death? 
Also, since I’ve given up being reasonable and rational - casting through your own flesh is definitely bad, but what happens if you cast through someone else’s flesh? The question arises because of the number of times people have wondered if the Raven Queen’s spells are limited by the body she’s inhabiting.
The problem comes because you can’t stop casting, right? So if you were to cast through a Null, for example, who can’t actually cast magic, what would happen? For example if Siobhan had to absorb some terrible spell via her shadow and her magic went completely haywire and all her conduits shattered. Would she be able to grab onto Oliver and channel that magic through him?
Yes, that does have the potential to be a very romantic scene if you ignore all the ethical implications. Why yes, I might have fan fiction percolating in my head. No, I’m not going to write it. Probably.
Probably collecting every romantic interaction between Siobhan and Oliver was a mistake. I have them on the brain now.
The floor on this level was polished marble instead of wood, and held a huge Circle with the symbol of an eleven-pointed star within. Beyond the boundary of the columns, several smaller Circles ringed the room.
“The building was originally built by a group of wealthy time-travel enthusiasts,” Deidre said with a wry twist of her lips. “They ended up massacring themselves in a magical accident about eight years ago. It was extremely gruesome, apparently, and word of the details spread quickly along with some rumors that the place retained traces of their magic, leaving people reluctant to buy it. Which is why the Undreaming Order was able to rent it for so cheap.”
Tosses more time travel evidence on the flaming pile of baseless speculation
that, and my eternal concern that doorways might not be what they seem.