Thomas Percy said:
I have another question, maybe silly, but it probably WILL happen when the PCs confront BigBadGuyNoctumancer: What will happen when a PC imprisoned in the Tomb (or Prison) of Night Shapechanges to size bigger than H, into the dragon exactly?
Hm... Actually there are two aspects to take into account:
1st:
Prison of Night /
Tomb of Night affects one creature of up to Huge size. Does the spell still affect the target if it becomes bigger than Huge?
I'd say, as the target was 'legal' (<= Huge) at the time the spell was cast, the spell stays in effect, even when the size of the creature changes later.
[It's like entering a room. If you enter the room through an open door you're in and it doesn't matter if somebody closes (and locks) the door later as you already
are in.]
[
Edit: The official FAQ seems to follow my reasoning:
official D&D FAQ said:
If, while under the effect of a spell that depends on type (such as hold person), my character is transformed into a different creature type by polymorph*, does the spell’s effect remain?
Yes. A spell only checks to see if you are a legal target when it is cast. If you become an illegal target later (such as via the polymorph spell), the spell remains in effect.
Guess you can apply that to the size-issue analogically.]
2nd: As the spells create some sort of actual 'prison' from shadowstuff, is there a chance of breaking that prison by becoming a creature bigger than the prison itself?
Like Ari I was quite sure the was a rule about what happens if a creature breaks confines of a room by shapechanging, but I'm at a total loss finding it.
However as
Prison /
Tomb of Night create some sort of 'mystical' prison out of shadow stuff and nothing physical (e.g. conjuring a prison made of stone), you shouldn't be able to break it (or else you would also be able to damage it by other means, e.g. hitting the prison with bludgeoning weapons). In the spell description a specific size of the 'prison' is not given, either.
Thus one approach would be to allow the shapchange with the spell remaining in effect, because the prison now kind of 'adjusts' to the new size (it's made of shadow stuff after all, and shadows can be quite flexible

).
To acknowledge the creativity of the player, the approach I'd take, however, would be to allow the shapechanging player another Fortitude save (or maybe even an opposed caster level check) to break out of the prison, out of the row. If successful, the shapechanged player breaks the prison.
[
As you begin to change shape you feel the grip of your shadowy prison tighten around you - The shadows flicker in an last attempt to hold you in their cold embrace, but finally they give away and fade into nothingness. You are free.]
If not successful the 'prison' suppresses the shapechange attempt and the spell remains in effect.
[
As you begin to change shape you feel the grip of your shadowy prison tighten around you. The shadows seem to drain the arcane power from you and finally an overhelming exhaustion forces you to end the shapechangeing attempt. The shadows proof to be too powerful for your puny attempt to challenge them. You remain a prisoner to the darkness.]