dnd 5th – Coming with a block of statistics for Cyrog from Out of the Abyss [Minor Spoilers]

This question contains spoilers for Out of the abyss, although spoilers are primarily for context (in the event that this would influence anyone's responses; that is, there is a reason based on a story for which Cyrog might have some capacity that I have not considered, etc.) and the heart of the matter is to create a block of statistics that does not exist in the published adventure.

Context

In Chapter 11, at Gravenhollow, the party spoke with

an "echo" of the past of a spirit flayer called Xetzirbor. I played him as suggested on page 154, asking him to talk about the demon lords with the plot, and very intrigued by what they were telling him, while hardly revealing anything outside of himself, except that he is there to prevent Cyrog from dying. In particular, when my players mentioned "Orcus", he was very interested and suddenly had to leave and end the conversation in a polite but abrupt way. The players said, "We should not have had to tell him all that," "Have we just revealed our secrets to a future villain or something?", Which pretty well announced the next one.

Later in Gravenhollow,

the PCs had a vision (among others), the one in particular that answers this question, "Cyrog Lives! Hail Orcus!" (p.158), which explains that Orcus appears next to the brain "Cyrog" and "bringing him back" recently deceased, so Cyrog tells the colony that "Orcus saved Cyrog" and commands him to "follow him" " [Cyrog] That's when the players were like, "Oh! ***, we caused that! I knew we should not have told him all that … ", and so on.

However, nothing more comes from this in adventure, so I wanted to extend the end of the adventure to include this. My plan is to extend the adventure to include Cyrog and his colony (assuming the players want it).

The result of all this is that I need statistics for Cyrog, who will now be a brain of undead undead, essentially a senior brain Lich.

Question

To establish the Cyrog statistics itself, I will use the Elder Brain statistics block as a starting point. I've changed the guy to undead and I'll have some traits and changes inspired by Alhoon.

For example, a standard Mind Flayer has 12 CON, but an Alhoon has 16; the elder's brain has 20 CON's, so if I increase that number to 24, it will have more life points about as much as the Alhoon on the standard Mind Flayer (I know that's the only thing I've ever seen). it's also a bit more likely to die, but I'm giving on that).

It should also have the Alhoon's reversal resistance feature, as well as control of the CON backup jets, as well as all damage and resistance to Alhoon conditions; these also seem to match the traits of a lich. However, I do not think I need to change the Old Master's brain spells (as tempting as giving him necromancy spells, he's still a psionic caster rather than a launcher and I do not think that would change that).

Just to recap the above:

  • Use seniors' brain statistics as a basis;
  • Creature type changed to undead;
  • The CON capacity score has been increased by 4 (which has an impact on PV);
  • Added control of the backup jets of the CON;
  • Added turn resistance feature (from Alhoon / Lich);
  • Damage resistance added: cold, lightning, necrosis (from Alhoon / Lich);
  • Added damage immunity: poison; non-magical knocking / piercing / notching (from Alhoon / Lich);
  • Immunities with added conditions: charmed, exhausted, frightened, paralyzed, poisoned (after Alhoon / Lich);

So, my question is twofold:

  • Is there anything that I have forgotten in the foregoing that a senior undead brain should have been ("should it" be defined as "what is it that would make it compatible with other intelligent creatures of the undead ")?
  • What would be the CR for this creature? Surely more than a standard CR 14 Elder brain; does that mean CR 15?

NB: I am aware of this, and I am sure that there are probably other homebrews for Cyrog, but I prefer what I have proposed so far.