What is the purpose of the prison at the ending?

As far as I am aware, the prison was made by Solas to contain the remaining elven pantheon before he destroys their old prison (the veil). It's powered by regret and has a built-in torture device designed to taunt the occupant(s) with their greatest failures in order to keep itself running.

At the end of my game Solas willingly walked into the fade with the inquisitor. I assumed at the time he was going to the black city to quell the blight, but Trick Weekes apparently stated* that it was their intent that he's returning to the prison. Why would he return to the torture nexus instead of... anywhere else in the fade? Especially if he's joined by the inquisitor? THEN I watched the three other endings where Solas is actively PULLED into the veil by the prison after stabbing them.

Why is the prison pulling him in / why is he returning there willingly? What am I missing?

The prison shouldn't care about Solas getting stabbed, Rook doesn't need to be cut by the dagger to get trapped there. I can't imagine Solas's plan was to individually stabbinate the gods to bind them to the veil to suck them in, when that would keep the veil alive and defeat the entire purpose.

*I can't find the bluesky...(tweets?) but here is a list of the Q&A.