SPOILERS:
in 9.1 it is 100% confirmed that the Primus and Runecarver are one and the same. And as some alluded the whole thing was a ploy to get the player to unlock the key of the Primus by using Runecarver himself to unlock the very thing he (Primus) locked away via "memory magic"
We haven't see the cinematic that plays out but it seems like the Jailer walks away with the key but somehow Primus is allowed to escape along with the player...
Why is the chamber so unguarded? Why not have legions of guards there waiting in the shadows for the right moment? Why does the jailer continously allow us to enter his halls with little to no resistance?