Bite

Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 32 (5d10 + 5) piercing damage plus 10 (3d6) radiant damage. If the target is Large or smaller, it is grappled (escape DC 17). Until this grapple ends, the target is restrained and the ammut can't bite another target.

Swallow

The ammut makes one bite attack against a Large or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained and it has total cover against attacks and other effects outside the ammut. An ammut can only have one Medium or smaller creature swallowed at a time. If the ammut takes 30 damage or more on a single turn from the swallowed creature, the ammut must succeed on a DC 14 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 feet of the ammut. If the ammut dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 10 feet of movement, exiting prone.

Ghost Breath (1/Day)

The ammut inhales the tortured spirits of undead within 30 feet. Each undead creature of CR 1 and lower in the area is automatically destroyed. All other undead must succeed on a DC 17 Wisdom saving throw or be incapacitated for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Absorb Spirit (1/Day)

The ammut attempts to absorb the spirit of a dead or undead creature in its belly. The creature must succeed on a DC 16 Wisdom saving throw or be absorbed by the ammut. A creature absorbed this way is destroyed and can't be reanimated, though it can be restored to life by powerful magic, such as a resurrection spell. The ammut regains hp equal to the absorbed creature's hp maximum.