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

Magic Weapons. The bag's weapon attacks are magical.

Multiattack. The bag makes two attacks with its tentacles, or it may make two bite attacks, or one attack with a tentacle and one attack with a bite, or one swallow attack. 

Tentacle. Melee Weapon Attack: +7 to hit, reach 15 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (2d6) necrotic damage. If the target is a creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the bag can't use this tentacle on another target.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 11 (2d6 + 4) piercing damage. The target must succeed on a DC 15 Constitution saving throw or take 10 (3d6) poison damage and become poisoned until the end of its next turn.

Swallow. The bag makes one bite attack against a Medium or smaller creature it is grappling. On a hit the target takes damage as if from a normal bite attack. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained (escape DC 20), it has total cover against attacks and other effects outside the bag, and it takes 21 (6d6) acid damage at the start of each of the bag's turns. The bag can have only one creature swallowed at a time.

If the bag dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Throw. The bag may use a bonus action throw a target restrained in its tentacles 20 feet. The throw target takes 2d6 bludgeoning damage or half on a successful DC 10 Dexterity saving throw. The bag may also throw restrained target at another target within range +5 to hit. Thrown target figures damage as above, hit target takes 2d6 bludgeoning damage or half on a successful DC 10 strength saving throw.

Spit. The bag may use a bonus action to spit up a swallowed player 15 feet away from it. The target takes 2d6 bludgeoning damage or half of a successful DC 10 dexterity saving throw.

Vile Retribution. When the bag takes damage from a creature within 60 feet of it that it can see, it can force that creature to make a DC 15 Wisdom saving throw. On a failed save, the creature takes 10 (3d6) psychic damage and has disadvantage on attack rolls and ability checks until the end of its next turn. On a successful save, the creature takes half as much damage and doesn't have disadvantage.