Wrythass

Worm of Wyrthass: A creature bitten by a worm of Wrythass (including his slam attack) has disadvantage on saving throws against spells being cast by him.  

Regeneration: Wyrthass regains 5 hit points at the beginning of each of his turns, if he was not attacked with acid in the previous round.

Spellcasting: Wyrthass is a 9 level spellcaster, using intelligence as its modifier. (Spell save DC 16, +8 to spell attack). Wrythass has the following spells prepared:

 Cantrips: Acid splash, poison spray

1 level (4 slots): Charm person, shield, fog cloud, alter self

2 level (3 slots): Detect thoughts, invisibility, melfs acid arrow, mirror image

3 level (3 slots): Slow, dispel magic, vampiric touch

4 level (3 slots): Banishment, hallucinatory terrain

5 level (1 slot): Telekinesis

Spider senses: Wrythass can climb walls and ceilings using his natural speed, and he can clearly see any creature obscured by spell effects such as darkness or fog.

Slam. Melee weapon attack + 4 to hit. Hit: 1d8 bludgeoning damage. Target must succeed a constitution saving throw of 12, or be overrun by worms, taking 1d6 poison damage, falling prone to the ground in the process.

Tentacle Rod: Wyrthass makes 3 attacks with this action. Ranged attack reaches 15 feet + 5 to hit. Hit: 1d6 bludgeoning damage. If all three hits, DC15 constitution save or speed is halved.