Antlion

Amphibious. The Antlion can breathe air and water.

Shock response: If the Antlion takes electric damage, it must use its reaction to Undermine. 

Multiattack. The Antlion makes two attacks with its tentacles. It can also use undermine or bite.

Tentacle. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage, and the target is grappled (escape DC 16) if it is a Huge or smaller creature. Until the grapple ends, the Antlion can’t use this tentacle on another target. The Antlion has four tentacles.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) piercing damage, and the target is swallowed if it is a Medium or smaller creature. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the Antlion, and takes 10 (3d6) acid damage at the start of each of the Antlion’s turns.

The Antlions’ gullet can hold up to two creatures at a time. If the Antlion takes 20 damage or more on a single turn from a creature inside it, the Antlion must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls prone in a space within 10 feet of the Antlion. If the Antlion dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 10 feet of movement, exiting prone.

Undermine. The Antlion targets all Medium or smaller creatures that it can see within 40 feet of it. The targets must make a DC 18 Strength saving throw. On a failed save, the target is pulled 10 feet directly towards the Antlion, if a target comes within 5 feet of the Antlion, the Antlion can make a bite attack against it as a bonus action.

the Antlion can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Antlion regains spent legendary actions at the start of its turn.

• Tentacle. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage, and the target is grappled (escape DC 16) if it is a Huge or smaller creature. Until the grapple ends, the Antlion can’t use this tentacle on another target. The Antlion has four tentacles.
  
• Sandball. Ranged Weapon Attack: +10 to hit, reach 80 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage