Yep, basically their weapon and/or arm freeze when they hit it... and makes it harder for them to hit it two rounds in a row. Also lets it maneuver around to affect more folks.
I think I'm tempted to switch icy prison to more like my original thought: have it have some hit points and block line of effect, instead of just being another save ends. Forces you to deal with it (or someone turns into a block of ice)
Yeah, I suspect the dragon will poof up to 5th-7th level when I get a monster manual, but for now 3rd at least makes it possible to try to test it out
I might also have to drop an ability or two and have them only on the higher level ones. For solo creatures, my instinct is to give more abilities so that the combat doesn't turn static, but I still probably need to reign things in.
I suspect a wall of ice like the wizard power would suit the ancient version well, assuming it's not over the top for number of abilities.