Multiattack. The demon makes two attacks: one with its tentacles and one with its bite.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) piercing damage.  If the target is a creature, it is grappled (escape DC 15).  Until this grapple ends, the target is restrained, and the demon can't bite another target.

Tentacles. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 12 (4d4 + 2) piercing damage.  If the target is a Large or smaller creature, it is grappled (escape DC 15).  Until this grapple ends, the target is restrained, and the demon can't use the tentacles on another target.  If this attack is a critical hit, the target can't breathe or speak until the grapple ends.

Swallow.  The demon makes one bite attack against a Medium or smaller creature it is grappling.  If the attack hits, that creature takes the bite's damage and 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 demon, and it takes 21 (6d6) acid damage at the start of each of the demon's turns.
If the demon takes 15 damage or more on a single turn from a creature inside it, the demon must succeed on a DC 15 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 demon.  If the demon dies, a swallowed creature is no longer restrained by it, and can escape from the corpse using 5 feet of movement, exiting prone.

Disgorge (Recharge 6).  The demon vomits in a 25-foot cone.  Each creature in that area must succeed on a DC 13 Dexterity saving throw or take 21 (6d6) acid damage and fall prone in the spew.