Blind to Stillness. The tyrannosaurus can only see things that move. It has a disadvantage on any Wisdom (Perception) check to see a non-moving creature.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit:  23 (3d10 + 7) piercing damage. If the target is a Small or smaller creature, it is grappled (escape DC 17). Until this grapple ends, the target is restrained, and the tyrannosaurus can't bite another target.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Intimidating Roar. The Rex roars. Each creature of the Rex's choice that is within 60 feet of the dragon and can hear it must succeed on a DC 8 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Frightful Presence for the next 48 hours.