Opalescent Shine. "Rex" has Advantage on saving throws against spells and other magical effects.

Undead Nature. "Rex" doesn't require air, food, drink, or sleep.

Undead Fortitude. If damage reduces "Rex" to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, "Rex" drops to 1 hit point instead.

Multiattack. "Rex" makes a Tail attack and either a Bite attack or Bone attack. It can't make both attacks against the same target.

Bite. Melee Attack Roll: +11, reach 10 ft. Hit: 33 (4d12 + 7) Piercing damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 19). While Grappled, the target has the Restrained condition and can’t be targeted by the "Rex's" Tail. Until this grapple ends, "Rex" can't Bite a different target or make a Bone attack.

Bone. Ranged Weapon Attack Roll: +11, range 30/60 ft. Hit: 20 (3d8 + 7) Bludgeoning damage. All creatures within 10 ft of the target must make Dexterity saving throw (DC 19) or take 5 (1d8) Bludgeoning damage from the ricochet.

Tail. Melee Attack Roll: +11, reach 15 ft. Hit: 25 (4d8 + 7) Bludgeoning damage. If the target is a Huge or smaller creature, it has the Prone condition.