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

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

Reflective Carapace. Any time the Giant Triffid is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the giant triffid is unaffected. On a 6, the giant triffid is unaffected, and the effect is reflected back at the caster as though it originated from the Giant Triffid, turning the caster into the target.

Magic Weapons. The Giant Triffid’s attacks are magical.

Regeneration. The Giant Triffid regains 30 hit points at the start of its turn if it has at least 1 hit point.

Multiattack. The Giant Triffid makes four attacks: one with its bite and three with its vines.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a medium or smaller creature grappled by the Giant Triffid, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Giant Triffid, and it takes 28 (8d6) acid damage at the start of each of the Giant Triffid’s turns.

If the Giant Triffid takes 25 damage or more on a single turn from a creature inside it, the Giant Triffid must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the Giant Triffid. If the Giant Triffid dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Vine. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) piercing damage. The target is grappled (escape DC 16). Until this this grapple ends the target is restrained. The Giant Triffid can grapple up to 4 targets at a time. 

Teleport: The Giant Triffid can teleport to a space it can see within 120 feet. 

Noxious Gas: (Recharge 5–6).Can emit Cloudkill as a reaction to a successful attack against the Giant Triffid as a natural ability without any spell casting focus or materials.

Harden Carapace: (Recharge 5–6). Can cast the spell shield on a successful weapon attack against it.

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

Vine Attack: uses one vine attack

Teleport: Can swap places with one of the Triffid Spawns and immediately make a vine attack against any target within range.

Triffid Spawn: (2 actions) Can spawn 2-12 Spawn 2d6 Triffid Spawns within 60 feet