In need for advice! (spoiler alert)

[M0bious]

Hello everyone!
I am currently in chapter 3 (almost at the end of it) and I have added the module The Harrowing (by making some small changes for it to fit better) to our campaign! The PCs entered a strange demiplane (which was part of a prophecy in our campaign) with hopes of gaining great power and knowledge to face the dangers that ,according to them, clearly are threatening Varisia as a whole (especially after the attack of Black Magga). Everything is going great and the plan is simple: the Pcs will finish the module and then continue RotR as normal, by paying a visit maybe to the hook mountain clanhold and end the hook mountain massacre..
I only got one problem..the last encounter of the harrowing plane adventure is

Zassrion:

an adult blue dragon!

I certainly like the idea of it, for the last encounter of an adventure that so far my players really enjoy and it seems legit for the lord of this demiplane to be as strong as Zassrion, but.. it has some drawbacks too.
My thoughts:

spoiler:

1)Zassrion will be the first dragon they encounter in this campaign and I fear that it will totally steal that from Longtooth(juvenile red dragon) in the first part of chapter 4 of Rotr!!
2) Even worse, Zassrion is an adult blue dragon, the same as the last dragon they will encounter and face with special K in the end! I dont know if its wise for me as a GM to introduce a threat like that so early.

So those are my considerations..Do you guys and girls think that they are important?? Furthermore, if yes, how do you think I should deal with this case and fix it??
I already got some ideas but I would really love to read your opinions on this matter first. Thanks in advance!

[Mudfoot]

I don't have the module so I don't know what would fit. But if you can, replace Z with something else of suitable capability. Probably an outsider, or more than one. Boss encounters almost always go better with mooks, so multiple opponents seems sensible.

[Oserath]

Could you just age down the Blue Dragon?

[Ivan Rûski]

Well, my group just finished The Harrowing. I ran it more or less as written, but I dropped it in immediately after book 3. If you are concerned with it lessening the impact of Longtooth and Ghlorofaex, my suggestions would be as follows:

Spoiler:

Switch out Zassrion with a younger dragon, and maybe pull a bait and switch with the color. Have him physically be blue, but use the stats of another color. Personally, I feel keeping Zassrion blue is somewhat important, as the dragon on the Harrow card is blue. But the energy type of his breath weapon could be changed with no problem and very little in the way of explanation. Maybe Sonnorae got his story wrong. Maybe she changed his color in the tale because she liked blue better. Maybe his stitching mortal flesh onto himself has changed him somehow.

[Tangent101]

There's a couple of possibilities.

Spoiler:

First, you could switch out the dragon with a Primal Dragon from Bestiary 2. A mature adult Crystal Dragon should be the same CR range. Alternatively, reduce the age of the dragon so its CR is two less... and then make it a Ravener (an undead Dragon). That's also in Bestiary 2.

[Mark_Twain007]

I have not read the harrowing, so I don't now if it would make any sense, but if you switched the dragon out for

spoiler:

a Glabrezu demon, it would be the same CR, and be a nice little preview to the pit lord demons they meet later. It would give them a good idea of how dangerous demons are, which could affect them trying let the pit lord go free.