(I'm quite alright with the save going to Kei, just saying :D)

Kellen Wis save: 4

Gerrard and Kellen find their minds assaulted and twisted, spawning potent delusions and provoking their bodies into uncontrolled action. Somehow Kellen keeps ahold of the squirming demon in his arcane grip.

Flying demon DC16 WIS Save: 24

Gerrard: roll a d10.

Kei may act!

Gerrard’s d10:  8

Kei 

One eye watches as the water-worm approaches but his main attention is on the ichor filled bird. Kei points at the vulture and sends 3 smoke coloured bolts of energy coursing its way. Meanwhile his other hand flits and waves at the worm. Swirling patterns form around what might be its head. He only hopes it has eyes.

A - cast EB at the bird (rolls in log)

BA - Quickened cast Hypnotic Pattern at the approaching worm. WIS17 save or become charmed and movement of 0. That's of course, if it has eyes.

Gerrard, fighting wild delusions, uses his action to make a melee attack against Kellen. (I'll let Gerrard roll that...) At the end of his turn he may make a fresh DC16 Wisdom Save to fight back his hallucinations.

Kei focuses on the bird grappled above them with three beams of crackling energy. It is in very bad shape.

The horrid bird squirms to escape Kellen's arcane grasp. DC17 STR check: 14

Kellen (fighting his own hallucinations): roll a d10

Gerrard:

In his own world of delusion,  Gerrard lashes out at what he thinks is the vulture, but it is so hard to tell…

Rapier:  Attack: 22 Damage: 9

Wisdom Save: 14

7

Worm WIS17 save: 11

Kellen, tormented by his own delusions, uses his action to make a melee attack against Gerrard. (Go ahead and roll that, Kellen...)

The approaching worm is unfazed by Kei's lightshow, even as the flying demon is still caught in Kellen's grasp.

Rurik aims his ballista at the grappled bird-thing. 

Kellen

(Am I still able to use my bonus action to squeeze the demon?)

Seeing a nasty looking monster swiping at him with a metallic claw, Kellen grits his teeth and retaliates, sweating profusely:

Dagger attack on Gerrard: Attack: 25 Damage: 5

(Damage roll in case I can crush the demon still: 23)

Kellen somehow hangs on to the bird, but cannot conceptualize squeezing it. (Kellen and Gerrard may not take any bonus actions or reactions for the immediate future.)

The tentacle-tongue worm snakes ever closer. It exhales a cone of acid. Everyone: make a DC 18 Dexterity saving throw, taking 44 acid damage on a failed save, or half as much damage on a successful one.

Rurik DC 18 Dex Save: 1
Flying Demon DC 18 Dex Save: 9

Rurik falls dead, screaming horribly.

The flying demon dissolves into black ichor.

The ship itself is badly scarred by the acid, the sails dissolving into ashen scraps, together with most of Rurik's re-rigged helm lines.

Kei may act. Gerrard may roll a d10

Gerrard:

Dexterity Save:  17

D10:  10

Gerrard dodges the worst of acid spray, but is still seared (half damage = 22 acid damage.) Nevertheless, he is still wracked by wild hallucinations.
He lashes out again at Kellen.

(7–8 The target doesn’t move, and it takes the Attack action to make one melee attack against a random creature within reach. If none are within reach, the target takes no action.)

At the end of his turn, Gerrard may make a fresh DC16 Wisdom Save to try to get a grip on reality.

Kei may act normally.

Gerrard:

A wave of multicolored slime drenches Gerrard from a passing albatross excretion that he barely manages to partially dodge.  Clenching his fist and yelling at the bombardier bird, the delusional noble concentrates on trying to pop the gigantic sloth-shaped balloon aberration before him!

Rapier:  Attack: 12 Damage: 10

Somewhere in the back of his mind, Gerrard knows something is wrong…

Wisdom Save:  10

Kei DEX save 10

Dex save: 11

Between the acid and Gerrard's ferocious attacks, Kellen collapses. 

(I'll go ahead and make the first death save just in case: 4)

Kellen and Kei are not as lucky as Gerrard in dodging the acid spray from the gargantuan wormlike horror that is almost upon them. The acid-scarred Kellen collapses as Gerrard lays into him with his rapier, a wild-but-pained look in his eyes. (Death save √)

The massive worm reaches the ship, tearing into the hull with four fat tentacles:
Tentacles Attack vs. ship: 27. Hit:26 bludgeoning damage.

The warship releases a volley of huge bolts at the worm:

Bolt Attack #1 vs. worm: 17. Hit: 14 piercing damage.
Bolt Attack #2 vs. worm: 24. Hit: 14 piercing damage.
Bolt Attack #3 vs. worm: 13. Hit: 16 piercing damage.

Gerrard: Roll a d10
Kei may act!