Adult Amargasaurus

Trampling Charge. If the Amargasaurus moves at least 20 feet straight toward a creature and then hits it with a skull whack 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, Amargasaurus can make one stomp attack against it as a bonus action.

Threat Display. As an action, the Amargasaurus can lower its head close to the ground, presenting the spines on the back of its neck to attackers.  While making its threat display, the Amargasaurus has disadvantage on attack rolls made to attack with its tail, but gains a +3 bonus to AC. Additionally, whenever a creature touches the Amargasaurus or hits it with a melee attack while within 5 feet of it, that creature takes 4 (1d8) piercing damage.  The Amargasaurus can end its threat display as a bonus action.

Skull Whack. Melee Weapon Attack: +11 to hit, reach 10 ft, one target. Hit: 32 (5d8 + 6) piercing damage.

Stomp. Melee Weapon Attack: +11 to hit, reach 5 ft, one prone creature. Hit: 30 (4d10 + 6) bludgeoning damage.

Tail Swipe. Melee Weapon Attack: +11 to hit, reach 10 ft, up to three creatures. Hit: 30 (5d8 + 4) bludgeoning damage.

Intimidating Presence (2/Short Rest). When attacked the Amargasaurus rears up showing its gargantuan body, the attacker must succeed on a DC 13 Charisma saving throw or be frightened for up to 1 minute, if the attacker is immune to being frightened then this reaction does nothing.