Broodstalker

Keen Hearing. The broodstalker has advantage on Wisdom (Perception) checks that rely on hearing.

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

Tunnler. The broodstalker can burrow through solid rock at half its normal speed and leaves a 10-foot-diameter tunnel in its wake.

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

Fey Ancestry. The broodstalker has advantage on saving throws against being charmed, and magic can't put the creature to sleep.

Multiattack. The broodstalker makes three slam attacks

Slam. Melee Weapon Attack: +9 to hit, reach 10ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. 

Bite. Melee Weapon Attack: +9 to hit, reach 5ft., one target. Hit:  18 (2d12 + 5) bludgeoning damage. If the target is a creature, it is grappled(escape DC 16) Until this grapple ends, the target is restrained, and the broodstalker can't bite another target.

Swallow. The broodstalker makes one bite attack against a Medium or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, 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 broodstalker, and it takes 21 (6d6) acid damage at the start of each of the broodstalker's turns.

If the broodstalker takes 20 damage or more on a single turn from a creature inside it, the broodstalker 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 5 feet of the broodstalker. If the broodstalker dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Acid Spray (3-6 Recharge). The broodstalker spits acid in a 15ft cone. All creatures in the space must make a DC 15 Dexterity saving throw or take 28(5d10) acid damage on a failed save, or half as much on a successful one.

The monster 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 monster regains spent legendary actions at the start of its turn.

Slam (Costs 1 Actions). The Broodstalker makes one Slam attack.

Lethal (Costs 1 Actions). The Broodstalker attempts to recharge it's Acid Spray. If it successfully does so, It can use it as a free action on top of it's legendary action. If the Broodstalker has not yet used it's Acid Spray, It uses the legendary action to do so instead of recharging.