Cryo Regisvine

Don't Run. The cryo regisvine deals triple damage when a PC isn't present within 15 ft. of it.

Corolla. A regisvine has a corolla at its base, starting the encounter with 1,000 temporary hit points and immunity to all damage. Fire damage is doubled whereas all slashing, bludgeoning, and piercing damage is halved (quartered via resistance if the attack is non-magical nor non-adamantine) against these temporary hit points.

• After these temporary hit points are reduced to zero and at the start of the next creature's turn (1), the cryo regisvine is paralyzed, all attacks automatically score and are critical hits, and its immunities turn into vulnerabilities.
• Next turn (2), the regisvine is no longer paralyzed, but attacks are still critical hits and it is vulnerable to damage.
• Next round (3), the regisvine regains two expended uses of legendary abilities, legendary resistances and lucky, and its corolla at the top of its head. Creatures with a reach of 20 feet or less have a -10 to hit against the regisvine. It can also now use abilities under the Corolla phase. 

Legendary Ability (5/day). If the regisvine fails something that would require a roll, it can choose to succeed instead.

Legendary Resistance (5/day). If the regisvine fails a saving throw, it can choose to succeed instead.

Lucky (10/day). The regisvine has 10 uses of the lucky feat, of which he can use multiple at once.

Oneshots. The creature treats immunities as resistance and invulnerability as independent constructs with 1,000 hit points (autohit, autofail). The creature also has expertise in saving throws against effects that would cause conditions. Don't be surprised when previous abilities were forgotten for the sake of concise text.

 expertise in all saves

Multiattack. The cryo regisvine makes three attacks, which can be Ice Beams, Homing Icicles, or Faceplant, but no more than two of each per turn.

Face Plant. Melee Weapon Attack: +25 to hit, reach 15 ft., 25 ft. cube. Hit: 62 (12d6 + 20) bludgeoning damage plus 78 (12d12) cold damage. Large or smaller creatures are knocked prone. Large or smaller creatures must make a DC 33 Constitution save or be stunned. A creature can give up its action to try to make the check with advantage to end the condition.

Homing Icicles. Ranged Weapon Attack: auto hit, range 180 ft., one target, three icicles. Hit: 78 (12d12) cold damage. A creature hit by two or more icicles has its movement speed reduced to zero until the start of its next turn.

Ice Beams (Recharge 3-6). Ranged Weapon Attack: +25 to hit, 100 ft. long, 10 ft. wide, two beams. Hit: 156 (24d12) cold damage.

If the cryo regisvine has regained its corolla within the past hour, it can use the following abilities in place of its attacks.

Face Smash. Melee Weapon Attack: +25 to hit, reach 15 ft., 25 ft. cube. Hit: 97 (22d6 + 20) bludgeoning damage plus 78 (12d12) cold damage. Large or smaller creatures are knocked prone. Large or smaller creatures must make a DC 23 Constitution check or be stunned. A creature can give up its action to try to make the check with advantage to end the condition.

Flail. All creatures within 15 ft. of the cryo regisvine must make a DC 23 Dexterity saving throw or take 97 (22d6 + 20) bludgeoning damage plus 78 (12d12) cold damage. For one minute, the area within 15 ft. of the cryo regisvine turns freezing. For one minute, each creature in the area must make a DC 23 Dexterity saving throw at the start of each of its turns or take 234 (36d12) cold damage.

The regisvine can take 3 legendary actions, choosing from the options below. Any number of legendary action options can be used at a time and only at the end of a creature's turn. If the regisvine has unspent legendary actions at the start of its turn, it may spend and immediately use its remaining legendary actions. The regisvine regains spent legendary actions at the start of his turn.

Attack. The regisvine makes an attack.