Undead Fortitude. If damage reduces the zombie 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, the zombie drops to 1 hit point instead

Trampling Charge. If the ultimasaurus moves at least 20 feet straight toward a creature and then hits it with a gore attack on the same turn, that target must succeed on a DC 13 Strength saving throw or be knocked prone. If the target is prone, the ultimasaurus can make one stomp attack against it as a bonus action.

Disgorge Zombie. As a bonus action, the ultimasaurus zombie can disgorge a normal zombie, which appears in an unoccupied space within 10 feet of it. The disgorged zombie acts on its own initiative count. After a zombie is disgorged, roll a d6. On a roll of 1, the tyrannosaurus zombie runs out of zombies to disgorge and loses this trait. If the tyrannosaurus zombie still has this trait when it dies, 1d4 normal zombies erupt from its corpse at the start of its next turn. These zombies act on their own initiative count.

Multiattack. The ultimasaurus makes three attacks: one with its bite, one with its tail, and one with it's claws. It can't make all attacks against the same target.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 33 (4d12 + 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.

Claws. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) slashing damage.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) piercing damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.

Gore. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 24 (4d8 + 6) piercing damage.

Stomp. Melee Weapon Attack: +10 to hit, reach 5 ft., one prone creature. Hit: 22 (3d10 + 6) bludgeoning damage.

Frightful Presence. Each creature of the ultimasaurus’s choice that is within 120 feet of the ultimasaurus and aware of it must succeed on a DC 19 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 the dragon’s Frightful Presence for the next 24 hours.

Rampage. When the ultimasaurus reduces a creature to 0 hit points with a melee attack, he moves up to half his speed and makes one Bite attack.

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

Charge. Ultimasaurus moves up to his speed.

Tail Attack. The ultimasaurus makes a tail attack.

Savage (Costs 2 Actions). Ultimasaurus makes a separate Bite attack against each creature within 10 feet of him.