Warforged Tyrannosaurus Rex

Berserk: Once the T-Rex hits fifty percent hp, it gains advantage on all attacks, but attacks against the T-Rex have advantage as well.
Dying Detonation: Once it hits 0hp, the T-Rex glows with elemental energy, exploding in a ball of multi-colored light in a 30-foot emanation from itself, dealing 4d12+10 damage. (Damage type determined by last type used in Elemental Laser Beam.)
Silent Stalker: The T-Rex has advantage on stealth checks.

Multiattack. The tyrannosaurus makes two attacks: one with its bite and one with its tail. It can't make both attacks against the same target. 

Bite. Melee weapon attack: +10 to hit, reach 10ft., 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. 

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

Chosen Prey: As a bonus action, the T-Rex's red eyes target a creature of its choice, gaining advantage on attacks against the selected creature until it changes its chosen prey. Chosen prey can be changed as a bonus action on each subsequent turn.

Legendary Resistances: 3

Legendary Actions: 3/round

Move (1 Action). The Warforged T-Rex can move up to half of its speed without provoking opportunity attacks.

Stomp (1 Action) The Warforged T-Rex smashes the ground with a thunderous stomp emanating 15-feet from it. Each creature within 15-feet must make a Dexterity Saving Throw (DC 15) or be knocked prone and take 2d8+5 bludgeoning damage. Success: Half damage.

Elemental Laser Beam (2 Actions; Recharge 5-6). Roll 1d4 with each use of this action and assign these elements to each number: Acid, Lightning, Fire, and Ice. The T-Rex fires a beam of elemental energy at a single creature it can see within 60ft. The creature hit must make a DEX saving throw (DC 15) or take 4d12+10 damage and is knocked back 15ft and has the prone condition. Success: Creature takes half damage and is not knocked back nor has the prone condition.

Mechanical Camouflage (3 Actions). The T-Rex's hide mirrors its surroundings, giving attacks against it Disadvantage until it takes damage. Once this happens, the camouflage fails, and the T-Rex returns to normal. Until the camouflage is disrupted, the T-Rex also has a +5 bonus to stealth checks.