This question contains spoilers for Out of the Abyss, although the spoilers are mostly for context (in case that influences anyone's answers at all; i.e. if there's a story-based reason why Cyrog could have a certain ability that I haven't considered, etc.) and the core of the question is about creating a stat block that doesn't exist within the published adventure.
During Chapter 11, in Gravenhollow, the party spoke with
an "echo" from the past of a mind flayer called Xetzirbor. I played him as suggested on page 154, having him grill the PCs about demon lords and acting very intrigued with what they were telling him, whilst barely revealing anything about himself besides that he's here to save Cyrog from dying. In particular, when my players mentioned "Orcus", he was very interested, and then suddenly needed to leave and ended the conversation politely but abruptly. The players were like "Maybe we shouldn't have told him all that stuff", "Have we just told our secrets to a future villain or something?", which foreshadowed the next bit quite well.
Later in Gravenhollow,
the PCs had a vision (among others), the one in particular that is revelent to this question, "Cyrog Lives! Hail Orcus!" (pg. 158), which details Orcus turning up next to the recently desceased Elder Brain "Cyrog" and "bringing it back", then Cyrog tells the colony that "Orcus has saved Cyrog" and commands them to "follow it [Cyrog] into undeath". It was at this moment that the players were like "Oh s***, we caused that! I knew we shouldn't have told him all that stuff...", etc.
However, nothing more comes of this in the adventure, so I wanted to extend the end of the adventure to include this. My plan is to extend the adventure to include dealing with Cyrog and its colony (assuming the players want to).
The upshot of all of this is that I need stats for Cyrog, who will now be an undead Elder Brain, essentially an Elder Brain Lich.
To come up with the stats for Cyrog itself, I will use the Elder Brain stat block from Volo's Guide to Monsters as a starting point. I've changed the type to undead, and will have a few traits and changes inspired by the Alhoon.
For example, a standard Mind Flayer has 12 CON, but an Alhoon has 16 CON; the Elder Brain has 20 CON, so if I increase this to 24, it will have more health by roughly the same amount that the Alhoon has over the standard Mind Flayer (I know it also has a few more hit die, but I'm overlooking that).
It should also have the Turn Resistance trait from the Alhoon, as well as proficiency in CON saving throws, and all of the damage and condition resistances that the Alhoon has; these seems to match up with the traits of a Lich as well. I don't think I need to change the Elder Brain's spells though (as tempting as it is to give it some necromancy spells, it's still a psionic spellcaster rather than an arcane spellcaster, and I don't think becoming undead would change that).
Just to recap the above:
- Using the Elder Brain stats as a base;
- Creature type changed to undead;
- CON ability score increased by 4 (which impacts HP);
- Added proficiency in CON saving throws;
- Added Turn Resistance trait (from Alhoon/Lich);
- Added Damage Resistances: cold, lightning, necrotic (from Alhoon/Lich);
- Added Damage Immunities: poison; nonmagical bludgeoning/piercing/slashing (from Alhoon/Lich);
- Added Condition Immunities: charmed, exhaused, frightened, paralyzed, poisoned (from Alhoon/Lich);
So my question is two-fold:
- Is there anything I've missed in the above that an undead Elder Brain should have ("should" being defined as "what would make it consistent with other intelligent undead creatures")?
- What would the CR for this creature be? More than a standard CR 14 Elder Brain surely; so does that mean CR 15?
NB: I'm aware of this, and I'm sure there's probably other homebrew out there for Cyrog, but I prefer what I've come up with so far.