Modifying a dragon without changing the CR

[Winternight]

Hi,
I have a dragon:
Black Dragon - Adult: CR 10; Large Dragon ; as MM
He has a caster level of 3.
Breath Weapon (Su): A black dragon has one type of breath weapon, a line of acid. 12d4 (DC23)
I want him to have a caster level of 10 or at least of 8.
To be precise I want him to be able to cast or have as a SP
-Polymorh self (or shapechange in human form only)
-scry
- cloudkill
- scare
- suggestion

What do i have to take away from the dragon to balance the CR more or less?
decrease his Breath Weapon ?
lower his hitpoints
Suggestions welcome

 

[Norfleet]

It's not a question of what you have to take away from the dragon. It's what you have to give to him as well.

For instance, the CR of a monster can be lowered if the monster happens to be on fire at the time the players encounter it. Since being on fire is disadvantangeous to the monster, he's somewhat less difficult. Other solutions include giving him a maimed wing, leg, or arm, or poking out one of his eyes, giving him an eyepatch, and making him a pirate dragon.

 

[Snipehunt]

That's going to be hard to pull off. Wouldn't it be easier to make some wondrous items as part of the dragon's hoard, and use those instead of adding spell casting? You could limit the uses of the item to lessen the impact on your campaign when the PC's eventually defeat the BBEG.

Think about this, maybe:

-a ring of polymorph self, only allows to change into human shape (once/day).
-cloudkill - maybe, a few scrolls? They won't always work for the dragon, but probably good enough.

I don't know if you need scare - the frightful presense ability already there isn't good enough? It's already DC20.

For suggestion, just give the dragon a wand. It's pretty much certain to make the caster check, and a wand of suggestion isn't going to hurt anything.

My 2 cp's.