I never liked the worm-that-walks as a monster. It just seems like something I would BURN WITH FIRE instead of negotiating with, which is somewhat expected in The Choking Tower with Nagrin Heruvex. I also have younger kids playing and was hesitant to show them the picture of Nagrin. So, reaching back to 4e days I pulled out the 4e lamia. This is very different to a standard lamia from lore and historical D&D. 4e lamia is basically an evil swarm creature but at least it looks pretty and the beetles don’t have the same icky factor that a body made of worms (maggots?) does. So updated it to 5e and replaced Nagrin with Nissa. It played out exactly the same way otherwise. Here is the fey lamia.
Armour Class 17 (natural)
Hit Points 216 (24d8 +24)
Speed 30 ft., 5 ft. burrowing
Initiative Speed +6 (Dex +2, Improved +4, spells -?)
STR 10 (+0) DEX 14 (+2) CON 12 (+1)
INT 13 (+1) WIS 17 (+3) CHA 18 (+4)
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, paralyzed, petrified, prone, restrained, stunned
Saving Throws Con +4, Wis +6, Cha +7
Skills Insight +6, Perception +6, Religion +4, Stealth +5
Senses blindsight 10ft., darkvision 120ft., passive Perception 16
Languages Elvish, Common
Challenge 10 (5,900 XP)
Fey Ancestry. The fey lamia has advantage on saving throws against being charmed, and magic can’t put the fey lamia to sleep.
Innate Spellcasting. The lamia’s innate spellcasting ability is Charisma (spell save DC 13). It can innately cast the following spells, requiring no material components.
At will: disguise self (any medium humanoid form), major image
3/day each: charm person, mirror image, scrying, suggestion
Spellcasting. The fey lamia is a 1Oth -level spellcaster. Her spellcasting ability is Wisdom (save DC 14, +6 to hit with spell attacks). The fey lamia has the following cleric spells prepared:
Cantrips (at will): guidance (C, +1d4 ability check), poison spray (CON 1d12/6), resistance (C, +1d4 save), spare the dying, thaumaturgy
1st level (4 slots): animal friendship, cure wounds (1d8 +6), detect poison and disease, ray of sickness
2nd level (3 slots): lesser restoration, protection from poison, web
3rd level (3 slots): conjure animals (2 giant beetles), dispel magic
4th level (3 slots): divination, freedom of movement
5th level (2 slots): insect plague (1zone, CON 4d10/20), mass cure wounds
Swarm Nature. By altering its shape, a fey lamia can squeeze through small openings as if it were a Tiny creature. Its damage output also drops by half once it reaches a bloodied state (1/2 hit points, 108 for Nissa).
Multiattack. The fey lamia makes two cursed touch attacks as well as a pacifying burst or devouring swarm attack (if available and able).
Cursed Touch. Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
Hit: 22 (6d6 + 4) necrotic damage and the fey lamia receives half this amount as healing. The target is magically cursed for 1 hour. Until the curse ends, the target has disadvantage on Wisdom saving throws and all ability checks.
Pacifying Burst (recharge on 5 – 6). The fey lamia psionically blasts everyone in a 30-foot cone. Each creature in that area must make a DC 16 Wisdom saving throw or become paralyzed. Save ends.
As a reaction if a creature is paralysed by pacifying burst the fey lamia can engulf the creature in a devouring swarm. She can use a bonus action to maintain the devouring swarm from round to round even switching targets so long as they are stunned.
Devouring Swarm. Melee Weapon Attack: +7 to hit, reach 10 ft., one target.
Hit: 13 (3d6 + 4) psychic damage.
The lamia uses disguise self to assume a pleasing appearance, hoping to lure enemies within range of its pacifying burst. It then immediately uses devouring swarm against a stunned foe within range. It sustains devouring swarm round after round as a bonus action, attacking a new target only if the previous target moves out of range. The lamia heals itself by making cursed touch attacks and unleashes another pacifying burst as soon as the power recharges.
Time permitting Nissa will conjure beetles to help in the battle and launch an insect plague. She will happily cast web and freedom of movement if this offers her a chance of escape as well.