Dunemaw Pit

Damage Resistances: bludgeoning, piercing
Damage Vulnerabilities: fire
Senses: tremorsense 120 ft., blindsight 60 ft., passive Perception 11
Languages: —
Challenge: 8 (3,900 XP)

Dunemaw Pit (CR 8)

STR 22 (+6) | DEX 10 (+0) | CON 22 (+6) | INT 1 (-5) | WIS 12 (+1) | CHA 4 (-3)

Abilities:

False Appearance: While buried beneath the sand, the Dunemaw Pit is indistinguishable from the surrounding terrain.
Swallow: A creature grappled by the Dunemaw Pit is pulled into its maw. If it succeeds on two consecutive grapple checks, the target is swallowed and takes 4d10 acid damage at the start of each of its turns. The creature is blinded and restrained while inside the Dunemaw Pit.
Lair Actions (if needed): The Dunemaw Pit's desert lair shifts around it, creating quicksand patches or forcing creatures toward its mouth.

Actions:

Bite: Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 3d12+6 piercing damage. If the target is a Medium or smaller creature, it must succeed on a DC 16 Dexterity saving throw or be grappled.
Tendril Grab (Recharge 5-6): The Dunemaw Pit can extend tendrils from the sands around it, grappling up to two creatures within 30 feet. Grappled creatures must succeed on a DC 16 Strength saving throw or be pulled toward the Dunemaw.
Sand Quake (Recharge 6): The Dunemaw Pit creates a localized earthquake, causing all creatures within 20 feet to make a DC 15 Dexterity saving throw. On a failed save, they take 4d10 bludgeoning damage and are knocked prone.

Legendary Actions (Optional for a CR 9 or 10 creature):

Tendril Pull: As a legendary action, the Dunemaw Pit can pull a creature grappled by its tendrils 10 feet closer to its maw.
Crushing Bite: As a legendary action, it can bite down harder on a swallowed creature, dealing an additional 3d10 piercing damage.