Opinions on this Megadoomer Outsider?

[Jergmo]

So, I warned my friend beforehand that there are many ways it could go terribly wrong, but he came up with the idea of a powerful being having been imprisoned inside of his character while he was in the womb and left it up to me to come up with it, should I be okay with it. He thought it would be interesting to have the possibility of this creature determining the ultimate fate of his character in the campaign.

The Outsider in question is mostly powerless in this state, but I thought a Glabrezu would be a very good candidate for the manipulation of the character, and I wanted to make it a rather powerful one. Originally it was going to have a lot of sorcerer levels, but I realized the way I was going it'd be powerful enough to simply break the imprisonment. Currently, it's 12 Outsider/8 Sorcerer, the base Outsider levels of a Glabrezu and will be CR 23 (34 hit dice total to play with). Would it be over-the-top to increase its racial hit dice into Gargantuan progression and make this bugger a titan of unholy doom etc.?

[Sinfire Titan]

Take a look at the Unholy Scion template in Heroes of Horror. You just described it.