Brachavyrm (Arm Demon)

Body Arms. The brachavyrm gains a pair of arms each time it consumes a Humanoid creature. For every five pairs of Humanoid arms the brachavyrm has, its speed is increased by 5 feet. A pair of arms wither and die after a month it was created.

Magic Resistance. The brachavyrm has advantage on saving throws against spells and other magical effects.

Spider Climb. The brachavyrm can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. The brachavyrm makes one Bite attack, one Constrict attack, and two Tail Stinger attacks. It can replace its Bite attack with a use of Swallow or Web Spray, if available.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 10 (1d10 + 5) piercing damage, and the target is grappled (escape DC 17). Until this grapple ends, the target is restrained, and the brachavyrm can't bite another target. 

Constrict. Melee Weapon Attack: +9 to hit, reach 5 ft., one Huge or smaller creature. Hit: 16 (2d10 + 5) bludgeoning damage, plus 16 (2d10 + 5) slashing damage, and the target is grappled (escape DC 17). Until this grapple ends, the target is restrained. The brachavyrm can constrict only one creature at a time.

Tail Stinger. Melee Weapon Attack: +9 to hit, reach 10 ft., one creature. Hit: 10 (1d10 + 5) piercing damage. If the target is a creature, it must succeed on a DC 16 Constitution saving throw or become poisoned until the end of its next turn.

Swallow. The brachavyrm makes one Bite attack against a Medium or smaller creature it is grappling. If the attack hits, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the brachawyrm, and it takes 11 (2d10) acid damage at the start of each of the brachavyrm's turns.

If the brachavyrm takes 20 damage or more on a single turn from a creature inside it, the brachavyrm must succeed on a DC 18 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the brachavyrm. If the tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 5 feet of movement, exiting prone.

If a swallowed creature does and the creature is a Humanoid, the corpse’s body is consumed and a pair of arms sprout from the brachavyrm’s body. When this happens, the creature can be restored to life only by means of a resurrection spell or greater.

Web Spray (Recharge 5-6). The brachavyrm spits webbing in a 30-foot line that is 10 feet wide. Each creature in that line must succeed on a DC 16 Dexterity saving throw or be restrained by webbing. As an action, the restrained creature can make a DC 16 Strength check, escaping from the webbing on a success. The effect ends if the webbing is destroyed. The webbing has AC 10, 5 hit points, vulnerability to fire damage and and immunity to bludgeoning, poison, and psychic damage.