Codex, the Elder Mother

Legendary Resistance (3/day). If Codex fails a saving throw, she can choose to succeed instead.

Slippery Creature. Codex has advantage on saving throws against being grappled and all dex saving throws.

Lively. At the beginning of each turn, Codex regains 11 (2d10) hit points unless Fire damage was taken.

Camouflage. Codex can spend an hour eating a corpse. When this is finished, she has the option of changing into their persona. This ability works like the spell True Polymorph. The new form lasts for 7 years and 7 days, until the vessel dies, or until dismissed. When one of these is reached, Codex climbs out of the vessel in her true form at full health.

Multiattack. Codex can use her Frightful Presence, then make 3 attacks- 2 with her forcipules and one with her tail.

Forcipule. A centipede's front legs are actually venomous fangs called forcipules, and are unique to centipedes. +15 to hit, reach 5 ft.1d10+9 slashing damage. On a hit, the target must make a DC 13 Con saving throw or take an additional 1d8 Poison damage and be Poisoned for three rounds. The target can repeat this save at the end of each of its turns, ending the effect on a success. Poison damage does not stack for each additional attack or failed save.

Tail. +11 to hit, 1d10+5 bludgeoning damage. Reach 15 ft.

Frightful Presence. Each creature that Codex chooses within 120 ft. of Codex and aware of her must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Codex’s Frightful Presence for the next 24 hours.

Quick Movement. Codex can take the Dash action as a Bonus Action

Evasive. Codex becomes immune to attacks of opportunity for one minute or until her HP is lowered below 50%. Can be used as a Bonus Action.

Tricky Catch. 3 times per long rest, Codex can use a reaction to automatically escape from a grapple or avoid being grappled. Otherwise, she has advantage on all saving throws against being grappled.

Codex can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Codex regains spent legendary actions at the start of their turn.

Tail Attack.  Codex can make a tail attack.

Repulsive. Any creature within a 30 ft radius of Codex needs to make a constitution saving throw (DC 13) or take 1d8 poison damage and fall prone, with no effect on a success.

Mother's Love. Codex can choose one hostile insectoid that she can see and force it to become friendly. The insect is then considered charmed by Codex, and Codex can command the insect to do something with a regular or legendary action. The creature cannot break free of its own will- however, can be persuaded by a former ally to make a DC 15 CHA save, upon succeeding that creature is no longer friendly to Codex and is immune to this effect for the next 24 hours.