Rundown: If the allosaurus moves at least 30 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 15 Strength saving throw or be knocked prone. If the target is prone, the allosaurus can make one bite attack against it as a bonus action.

Multiattack. The tyrannosaurus makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 19 (2d12 + 7) piercing damage. If the target is a Medium 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.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) slashing damage.

Tail. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 11 (1d8 + 7) bludgeoning damage.