AT-Rex

Immutable Form. The construct is immune to any spell or effect that would alter its form.

Magic Resistance. The construct has advantage on saving throws against spells and other magical effects.

Magic Weapons. The construct's weapon attacks are magical.

Cumbersome. The construct's can't take reactions and has disadvantage on Dexterity saves.

Legendary Resistance (3/Day). If the construct fails a saving throw, it can choose to succeed instead.

Multiattack. The construct stomps on any target that's in range. Then makes either a combination of two Twin Blaster or Light Twin Blaster attack. If the construct has Terrible Bite ready, then it can make this attack against one target. The construct can make four Little Arms attacks instead of Twin Blaster or Light Twin Blaster attacks as long the targets are huge or larger.

Stomp. Melee Weapon Attack: +15 to hit, reach 20 ft., 10 foot cube. Hit: 52 (8d10 + 8) bludgeoning damage.

Little Arms. Melee Weapon Attack: +14 to hit, reach 10 ft., 1 target. Hit: 34 (6d8 + 7) bludgeoning damage. Only can target creatures that are huge or larger.

Twin Blaster. Ranged Weapon Attack: +12 to hit, range 150/300 ft., 1 target. Hit: 28 (4d10 + 6) Radiant damage.

Light Twin Blaster. Ranged Weapon Attack: +12 to hit, range 200/400 ft., 1 target. Hit: 24 (4d8 + 6) Radiant damage.

Terrible Bite (Recharge 6). Melee Weapon Attack: +15 to hit, reach 20 ft., 1 target. Hit: 52 (8d10 + 8) Slashing damage. A creature that's reduced to less than 75 hit points or less by this attack must succeed a DC 18 Constitution or they are rent in half and die.

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

Roar. The construct makes a terrible roar, and all creatures within 120' must succeed a DC 15 Wisdom saving throw or be frightened of the construct for 1 hour. A creature that succeeds this save is immune to this effect for 24 hours.

Blaster. The construct makes a Light Blaster Attack.

Charged Particle Cannon (Recharge 5-6). (2 Actions) The construct opens it's mouth, drawing in particles, and then releasing them in a powerful beam at a single target with 300'. The target and any creature within 5 feet of the target must succeed on a DC 21 Dexterity saving throw or they take 43 (5d10+15) Fire damage and 43 (5d10+15) Radiant damage, half as much on a save. A creature that takes damage from this attack is also treated to have the spell Dispel Magic cast on them at 7th level.

Tail Attack. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 21 (2d12 + 8) bludgeoning damage.