Omen's initiative: 13

Jimena: Initiative 7

Intelligence: Nature 16

Sir Jimena recognizes the monster as a Tendriculos, a plant-based monster related to a shambling mound. The tendriculos seems to manifest in places were nature has been corrupted by dark magic. It is best known for possessing an extremely rapid regenerative ability, allowing it to quickly grow and regrow its body, with whole new leaves and vines appearing on it within just a few minutes. It cannot move very fast, but it has a long range on its tentacle-like vine attacks that it uses to snare prey, "swallow" them whole, and absorb nutrients from them as they digest them inside their "bodies."

It is also the Tendriculos' turn. It drops Sir Glint into its gaping maw and engulfs him, it's tentacle-vines now free to attack the other PCs.

It is Sir Omen's Turn.

Omen yells out "NO!" and instinctively stretches his right hand forward and 2 bolts of energy fire from his hands towards the target.

Bolt 1: 23          Damage (if hits): 1

Bolt 2: 24          Damage (if hits): 7

The first ray misses as the creature's form is not constant. The second connects burning a small patch of dead plant around it (assuming that's Fire Bolt?). blasting the monster, but leaving no visible damage.

It's Sir Jimena's turn.

sorry

Eldritch Blast

Jimena's Turn: Round 1

Jimena is nudged awake from the state of dissociation when she hears the warning from Sir Omen, and she responds as quickly as she can. She will draw out her warhammer and leap forward to engage in a melee combat and attempt to strike the plantation with her shield. She attempts to contact with Sir Glint: 

"Sir Glint, do you hear me? We'll rescue you, hold on!"

Action: Attack [Warhammer] x Tendriculos: Attack: 18 Damage: 5

Attack: 19 Damage: 8

Bonus Action: Shield Master Shove x Tendriculos: Strength (Athletics) 18

Special Action: Psionic Strike: Additional 2 Force Damage for the second attack

Sir Jimena leaps forward attacking the monster, becoming an engine of war. Both of her attacks hit, smashing pieces of plant matter into pulp. She finishes by shoving the beast with her shield (Tendriculos' challenge: 17; are you pushing it back 5 feet? Or attempting to knock it prone?)

(If the creature has immunity to the prone condition, then let's just shove it 5ft away. It's an experimental action.)

(It's not immune to prone)

The monster is surprised by the ferocity of Sir Jimena's attack and is knocked prone.

Sir Glint's turn consists of a Constitution Saving Throw: 8

The two knights can hear a muffled scream from inside the monster as it's constantly churning plant matter seems to be grinding at Sir Glint from the outside. 

The damage that Sir Omen and Sir Jimena dealt seems to be repairing itself as they watch, though a little is left behind. The creature "stands," which is really more of a reshaping of itself, and attacks Sir Jimena.

Tentacle 1 Attack: 23 Damage: 11
Tentacle 2 Attack: 27 Damage: 20

(oof)

Sir Jimena is also grappled, but the monster cannot eat you as it's currently still eating Sir Glint.

It is Sir Omen's turn.

(changed the post)

Omen draws his Long sword and grips it with 2 hands, making an attack at the creature.

Hit: 7

Damage if hits: 8

Sir Omen's blade skips off the creature's bark skin (it's AC is 15), but deals no damage. 

It's Sir Jimena's turn, you're grappled so you can't move without escaping, but you can still attack and do anything else.

Reaction: Jimena uses her psionic power to create a protective field around her and reduce the total amount of damage taken by 3.

Jimena's Turn: Round 2

Bonus Action: Second Wind: Jimena uses a bonus action to regain 6 HP.

Action: Jimena strikes again, twice with her warhammer to target the tentacles that have been keeping her grappled. At the same time, she will send a warning: "Sir Omen, the plant is regenerative, it's a result of dark magical effect."

Attack [Warhammer]: Attack: 25 Damage: 14

Attack: 24 Damage: 10

Sir Jimena again lands two walloping blows against the tendriculos. The monster is eerily silent apart from the slight grinding and slithering sounds of plant on plant as it moves, but there is no cry of pain or reaction on the faceless creature.

Sir Glint's Constitution saving throw: 3

Much of the damage Sir Jimena inflicted seems to heal itself again, before the tendriculos throws Sir Jimena away from itself. Please make a Dexterity Saving Throw (DC: 11) or take (28 bludgeoning damage and be knocked prone 20 feet away; on a successful save, you take half that damage, rounded down, and are not knocked prone, but you're still 20 feet away).

Dexterity Saving Throw x Full Damage: 4

Jimena drops to 0 HP.

Sir Omen's Turn

As his bonus action Omen will look to Jimena, a stange light will surround his head then Jimena. "Be healed" Omens says and Jimena receives 5 hit points healing from the light.
(Warlock Celestial feature)

Omen will make an attack on the creature with his long sword.

Hit: 11
Damage, if hits: 9

Jimena's Turn: Round 3

"Thank you for saving my life, Sir Omen. I owe you..."- Jimena stops in the middle of the sentence when she finds her fellow knight moving dangerously close to the creature. She will immediately use her psionic power to move the Tendriculos 30 ft away from Sir Omen. (Does the movement trigger attack of opportunity?)

Action: Telekinetic Movement x Tendriculos

Bonus Action: Regain an expended psionic die

Special Action: Action Surge: Drink one dose of Potion of Healing to restore 6 HP.

(No, in 5e, unless otherwise specified by the ability, movement that is forced upon a creature does not invoke an attack of opportunity. So being pushed, dragged, flung, etc. does not trigger and AoO).

Sir Glint's turn consists of a Constitution Saving Throw: 13. (You have stopped hearing his muffled screams)

The monster does not seem to have much of a mind... which makes sense as it almost certainly does not have any organ recognizable as a brain. It also does not appear to have eyes and reaches out it tentacles in an almost tentative manner, as though trying to feel the area, in a vaguely confused manner.

Sir Omen, it is your turn.