Brachiosaurus

Siege Monster. The brachiosaurus deals double damage to objects and structures.

Unstable . If the brachiosaurus takes an accumulated 50 points of damage during one use of its Bipedal Stance, it falls prone.

Multiattack. The brachiosaurus makes three stomp attacks

Bellow. Every creature within 120 feet of the brachiosaurus that can hear it must succeed on a DC 17 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 becomes immune to the brachiosaurus's Bellow for the next 24 hours.

Bipedal Stance (Recharge 6). The brachiosaurus stands up on two legs. In this state, its walking speed is reduced by half, and it has disadvantage on all Dexterity checks and saving throws. The stance ends if the brachiosaurus is incapacitated, falls prone, or chooses to end it as a bonus action.

Mega Stomp (Bipedal Stance Only). The brachiosaurus slams down on the ground below it, causing shockwaves in the earth below it. Each creature within 15 feet of the brachiosaurus must make a DC 18 Strength saving throw or take 55 (10d8 + 10) bludgeoning damage, or half as much damage on a success, bypassing resistance. A creature that fails the save by 5 or more is knocked prone. After taking this action, the brachiosaurus returns to its normal stance.

Spit. Ranged Weapon Attack: +5 to hit, range 30/120 ft., one target. Hit: 29 (4d10 + 7) bludgeoning damage plus 17 (5d6) acid damage.

Stomp (Normal Stance Only). Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 27 (3d12 + 8) bludgeoning damage. 

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

Kick (Normal Stance Only). Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 22 (4d6 + 8) bludgeoning damage and the target must make a DC 18 Strength saving throw or be pushed 10 ft. away from the brachiosaurus. The target also falls prone if it fails the saving throw by 6 or more.

Quadruped Stance. The brachiosaurus ends its Bipedal Stance, if it is active.

Spit. The brachio makes one spit attack against a creature within range.