- with the eating of a new brain of a humanoid with a level higher than an older brain, you're saying the older skills "fade" by a level...? Might be a good way to have them slowly disappear...?
No, it'll "fade" to a set lower level not by 1 level. If we have it lose 1 level per week or something it would be a nightmare to keep track.
My basic idea was it can any number of abilities from low-level brains but only one set of abilities from a high-level brain.
Hadn't quite decided on the levels when I wrote it, but I'm thinking 1st to 3rd level for the former, so 4th+ for the higher level brain.
So, if it eats multiple brains from humanoids of 4th+ level it gains all the class abilities of the class with the most levels (with the most recent brain winning ties), but it can only gain class abilities up to 3rd-level from other brains (the gu'en-deeko still gains all the Skill proficiencies that 4th+ brains have regardless of whether they are the highest level or not, which may more Skills than a 3rd-level character would have.)
I chose 4th level as the cut-off point because that's where classes start to grant Ability Score Advancements, and it didn't seem prudent to allow the gu'en-deeko to get that from multiple brains.
It would also make sense it the "increased maximum hit points" ability kicks in at 4th-level too, so it gets a permanent +1 hp for a 4th-level brain, +3 for a 6th-level and so on.
- maybe rather than automatically go insane, make it a chance? A DC 10 WIS check per brain of victim 7th level or higher? Or DC 10+1/lvl above
7th? If forgone conlusion, the ape might never eat a strong victim, but if a chance then it might risk it for power....?
Well maybe gu'en-deekos are so driven to eat brains they don't care if it'll drive them mad?
However, a save is fine by me. Although if we allow a save I'd have it "kick in" at the Gu'en-Deeko's Hit Dice of 6 rather than more than the monster's HD.
I'd make it class-level and mental ability score based.
Maybe set the DC to 10 plus the "brain donor's" proficiency bonus plus its highest mental ability bonus? So a 6th-level
Druid requires, say, a DC 14 save (10 + 2 Proficiency +2 Wis), a
Captain a DC 15 save (10 + 2 proficiency +3 Cha) and an
Archmage a DC 19 save (10 +4 proficiency +5 Int).
I'm tempted to have a risk of personality-overlay from any brain of 4th+ level, but with a lower DC?
We could have it level-based I guess, but 10+level seems a bit too high and I'd like a mental ability in there.
If the DC were, say, 6 plus level plus highest mental ability modifiers that'd make a standard 4th-level
Druid DC 12 (6 + 4 + 2) and an
Archmage DC 29! (6 + 18 + 5).
It's more usual to have it based on Proficiency Bonus though, especially as many NPC humanoids don't have levels listed, such as the aforementioned
Captain, who's 10d8+20 hit points suggest 10th-level, the three attacks per round would require an 11th+ level if a fighter, but the piddling Challenge 4 suggests only 4th level or so.
So perhaps use the standard spell formula of 8 + proficiency + ability bonus for 4th+ level brains, but use 150% the proficiency bonus if the Challenge/Class Level equals or exceeds the Gu'en-Deeko's Challenge/Hit Dice/Highest Class Level?
I'm also thinking it'd be desirable to to stat up a sample of a brain-boosted Gu'en-Deeko and it seems appropriate if it's a "Gu'en-Deeko Mage" who has brain-devoured a 6th-level wizard and a 2nd-level dwarf thief.
Then, purely by coincidence, the creature would approximately match Thaaak's brain-abilities from
The Halls of Tizun Thane.