hong said:
The REAL answer, IMO, is not to write adventures that depend on these artificial restraints on characters (PC or NPC). Would you write an adventure that assumed a PC was going to be captured at one point? Probably not; not only because players don't like having their characters being caught, but also because high-level characters have lots of options. Artificially limiting these options makes for a contrived feel. That goes for PCs and NPCs both.
I think this is poor advice.
The story should be what is important, not the limited spells and capabilities in the game. The only reason it feels artificial is that there is no 6th level Prevent Polymorphing spell in the book. If there was such a spell, then the question would probably have never been asked.
One could have an important Mayor in a city whose powerful Wizard sister was captured and I doubt most people would say that no, you should not have a story line where a powerful NPC is captured.
The only difference between these two scenarios is that it is easier to think of ways to prevent a powerful Wizard from casting spells than it is preventing a powerful Dragon from using an innate ability.
Several solutions come to mind:
1) Dragons have been around a long time. Just like people have ways to imprison other people, even powerful Wizards, Dragons would have developed a spell to imprison their enemy dragons as well. So, you are the GM, make one up. You do not even have to fill in the details, just do it.
2) Maybe there is an inactive volcano or something in the world where Dragon Magic does not work. Drop an unconscious Dragon into it and they are weak as a kitten, cannot Fly, cannot Polymorph, cannot Burrow, cannot Climb, etc.
But, do not limit your thinking to the spells available in the book. It does not have to be a spell. Or, it could be a spell or item which does not exist, but you can have it exist in your campaign. You are the GM. Bam. Dragon captured and imprisoned. You really do not even have to say how the other Dragon did it.