Somewhat Unique CR 12 Boss Monster

[ReagentMetatron]

I'm working on a one shot for a bunch of Level twelve characters for this weekend. The final Encounter is going to involve a pretty odd monster. The idea is that it's acting as the defense system for the core of a Hellfire Engine that the players need to destroy. The monster itself is a huge stationary Construct that shifts between three states. The basic details are as follows.

As a move action, the Core shifts to any one of the two states it is not currently in. This is an action it must take every round. It cannot linger in a form.

Rage State: In Rage State, the Core absorbs all spells cast on it, as though surrounded by an Anti-magic cone. It can cast spells that do either Fire or Electricity damage in this form (up to 4th level).

Sorrow State: In Sorrow State, the Core deals back half of all melee damage dealt to it back on the user. A Fort save halves that damage, making it a quarter overall. The Core can also summon devils to assist it, (though I'm still working on which devils, specifically, and how many/easily.)

Joy State: In Joy State, the Core absorbs all healing spells cast, taking them away from the recipient. A Will save for the caster splits this between the target and the core. The core can then heal itself up to its absorbed points as a standard action.

I'm looking for thoughts and criticisms, as well as advice for the blank spots. This is my first Homebrew.

[DracoDei]

Ok, first thing... a BOSS to go against a level 12 party should be HIGHER than CR 12! I think Party Level +4 is recommended for characters above a certain level... so CR 16 if that is correct. Of course. that might be for the "Pull out ALL the stops, kill half the party, even if they win" final combat of an entire campaign... but at party level is going to be rather anticlimatic.

[ReagentMetatron]

Ah, yes. Well, I didn't really mean CR 12 in the literally sense, so much as I meant it as being a hard challenge for ECL 12 characters.

[Magikeeper]

Could we have some basic details of your players?