Brain-Eating. If the gu'en-deeko spends 10 (2d3 + 6) minutes* eating the brain of a freshly killed humanoid (one who died less than 30 minutes previously), it gains the memories and many of the abilities of that humanoid one hour later. A gu'en-deeko can absorb skills, knowledge-based racial abilities such as Dwarven Combat Training, languages and most class abilities from the brain, but does not gain any physical traits or racial abilities of the humanoid whose brain it ate such as Darkvision, racial Ability Score Increases, or Dwarven Resilience.
 *If it matters, the exact number of rounds it takes the gu'en-deeko to crack open a dead humanoid's skull and devour the brain can be determined by rolling 3d12 + 1d8 + 76.
 If a gu'en-deeko absorbs the brain of a humanoid with a level 3 or lower and a Challenge Rating of 1 or less, the skills, racial abilities, traits and class abilities it absorbed from this low-powered brain last for 1 year. The gu'en-deeko retains the brain's memories after the year ends, but none of the powers it gained. A gu'en-deeko can possess the powers of up to five low-powered brains, if it eats a sixth the new brain replace the oldest of the low-powered brains in the gu'en-deeko's repertoire.
 If a gu'en-deeko eats the brain of a humanoid with a level of 4 or higher or a Challenge Rating of 2 or more, the powers it gains last until it eats the brain of another humanoid whose level or Challenge Rating is the same or higher. If that happens, the gu'en-deeko gains the powers of the new brain and those powers it gained from the previous high-level brain are reduced to a 3rd-level character of that class, then vanish a year later (as above).
 If a gu'en-deeko eats the brain of a humanoid with a level of 6 or higher, a Challenge Rating of 3 or more, or a mental ability (INT, WIS, CHA) of 16 or higher, the gu'en-deeko must succeed at a Wisdom saving throw against a DC equal to 10 plus the Proficiency Bonus and highest Mental Ability Bonus of the humanoid the brain came from or go insane, believing itself to be the humanoid whose brain it devoured.
 A gu'en-deeko's Charisma score increases to match the highest mental ability score from the brains it has eaten. If a gu'en-deeko acquires the class feature Ability Score Improvement from a stolen brain, it always invests the improvement in a physical ability score (STR, DEX, CON) rather than a mental one, regardless of how the humanoid it stole it from invested that class feature.
 Powers gained via brain-eating never stack; if the gu'en-deeko gains the same ability multiple times, only the most powerful one applies. For example, if a gu'en-deeko ate the brains of a 2nd-level cleric, a 3rd-level bard, and a 3rd-level druid, it gains 3rd-level Spellcasting, not three separate sets of Spellcasting (see below for Gu'en-Deeko Spellcasting). A gu'en-deeko can possess multiple abilities that use spells if they are different types of magic-use. For example, it could possess Innate Spellcasting from multiple sources, such as a Drow Elf and a Svirfneblin, together with Spellcasting from a Wizard and Pact Magic from a Warlock, all at the same time. A gu'en-deeko is unable to gain class features granted by an external power without that power's permission. Thus, if a gu'en-deeko ate the brain of a warlock, the gu'en-deeko would have to strike a bargain with that warlock's otherworldly Patron to gain most of the warlock class abilities.
 If the gained power is controlled by a mental ability (INT, WIS, CHA), it uses the numerical modifiers of the humanoid it was stolen from rather than the gu'en-deeko's. A gu'en-deeko that eats the brain of a druid with Nature +6 will gain Nature +6, not the Nature +1 it would have from its own Intelligence score and Proficiency Bonus.
 If the gained power is controlled by a physical ability (STR, DEX, CON), the gu'en-deeko can use its own ability score bonuses if this would give it a better numerical modifier than the humanoid it was stolen from.

Increased Hit Points: The gu'en-deeko gains bonus hit points from brain-eating. It either gains bonus hit points that give it a hit point total equal to the highest hit points of a humanoid whose brain powers it currently possesses, or it has hit points equal its own hit points (normally 45) plus 1 hit point per Hit Dice possessed by the humanoid with the most HD whose brain it has ever eaten, whichever is higher.

Challenge Rating: A gu'en-deeko normally uses its own CR or the highest CR of the humanoids whose brain powers it possesses, whichever is greater. If the gu'en-deeko has powers from numerous brains that effectively synergy together this might increase its CR at the DM's discretion (use the Dungeon Master's Guide section on monster creation for guidance, pages 273 through 283).

Spellcasting: If the gu'en-deeko eats one or more brains with the Spellcasting power, it uses the highest spellcasting level, spell save DC, to hit with spell attacks and number of spell slots among the brains it has eaten. Its spellcasting ability is Charisma. A gu'en-deeko with Spellcasting cannot change the spells it has prepared like a cleric or wizard, but can only cast the spells the humanoid spellcasters whose brains it ate had prepared at the moment of their deaths. If a gu'en-deeko loses access to a brain's Spellcasting power, all prepared spells from that brain become one-use, fading from the gu'en-deeko's prepared spells list as soon as it casts them. If losing a brain's Spellcasting power causes a gu'en-deeko's number of spell slots to fall, the gu'en-deeko does not lose the spell slots immediately, but its surplus spell slots become one-use and are lost once they're used to cast a spell. If this causes the gu'en-deeko to lose all ability to cast spells of a given level, it also loses all prepared spells of that level.
 

Multiattack. The gu'en-deeko makes two fist attacks.

Fist. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage.