The waterlogged corpses clumsily try to evade Peri's spell. 

(DC14 Dex Saving throws, clockwise from the doorway:)
Drowned Blade 1: 1 
Drowned Blade 2: 7
Drowned Blade 3: 1
Drowned Blade 4: 15
Drowned Blade 5: 9
Drowned Blade 6: 19

(Any attack roll against an affected creature or object has advantage if the attacker can see it.)

Boko is up next!

Boko looks up at the charging undead and quirks her head, and waves her hand. “No,” she says.

She casts stone shape on the doorway and attempts to make the doorframe stone and secure (first choice) or much smaller (second choice). New spell so defer to you on this.

She will wildshape with BA pending result of this spell…

The walls of this building are made of mortared masonry, and the floors are paved with flagstones, so there is plenty of stone here. The only things that are wooden are the doors and the beams.

Boko squeezes in between Peri and Gerrard to touch the stone wall. Under Boko's command, several feet of wall and floor melt and become plastic. She coaxes the wall into restricting the door opening into an irregular hook-shaped aperture. 

It will now take an action for medium-sized creatures to wriggle through it. (Free for you small folks.)The stone is AC 17 and has 18 hit points.

(Had the door been made of stone, she could have fully sealed it. But it was a wooden door, now smashed to splinters.)

Boko may still have her Bonus Action. 

One of the (violet-glowing) barnacle-encrusted figures squeezes into the room, its eyes fixed on Gerrard, its rusty sword drawn. It is covered in those grotesque blue boils you formerly saw on the survivors in the hole.

The others hammer away at the stone barrier from the other room.
Rusted Longsword Attack: 11. 5 slashing damage.
Rusted Longsword Attack: 9. 9 slashing damage.
Rusted Longsword Attack: 23. 6 slashing damage.
Rusted Longsword Attack: 13. 5 slashing damage.
Rusted Longsword Attack: 12. 8 slashing damage.
Rusted Longsword Attack: 8. 10 slashing damage.

Boko may bonus-act, and Thwack, Qivys, and Gerrard may act. (Advantage on attacks against the purple-glowing ones for the next 10 rounds or as long as Peri can keep his concentration up.)

Boko's stone seal has been smashed away with some powerful blows from the corpses. (But it did buy you all an extra round!)

Gerrard:

Gerrard puts his flaming dagger and glowing rapier to work against the creature before him, stabbing and slashing in the hopes that this undead and perhaps diseased monstrosity may be put down.  Somewhere in the back of his mind he wonders if, given the blue boils on these creatures, he is actually fighting the corpses of original inhabitants here...

Flaming Dagger:  Attack: 26 Damage: Unable to parse dice roll.

Flaming Dagger:  Attack: 25 Damage: 16

Glowing Rapier:  Attack: 22 Damage: 8

Not being able to get into the fray due to the lack of space Qivys will take his actions to:
action 1: cast Bless on those up front, Boko, Gerrard and Peri. 

Action 2: if Qivys can get a clear shot, he will fire a crossbow bolt at a target.

Attack: 19
Damage, if hits: 8

Thwack 

"Whoa" Boko's manipulation of the stonework leaves Thwack in awe for a split second. Only the flash of Gerrard's illuminated bladed draws his attention back to the immediate problems. He claps his claws together twice in the direction of the single corpse that squeezed into the room.

A - Attack: 28 Damage: 11

BA - Attack: 26 Damage: 7

Sneak attack damage on first hit 9

(Psychic blades attack and bonus action attack)

The creatures are slow, zombie-like, and easy to hit. Gerrard strikes with all three of his well-aimed swings. Even Qivys' sloppy shot with his crossbow manages to sink into the staggering corpse. Thwack's first shimmering psychic blade finishes it off. It collapses, inert, in the shattered, mutated doorway.

Then, Thwack's second psychic blade buries itself in the mind of the next corpse that staggers forward to take its place (#6.)

Boko may take her unused bonus action if she wishes.

Then, Peri and Boko (both blessed) may act. Five advancing corpses remain, three glowing an unnatural violet from Peri's spell. (Advantage on attacks against them.)

Boko is too frustrated by the failed use of stoneshape to use her bonus action. On her next turn she casts [spell]moonbeam[/spell right where the monsters are coming in, on their side of the door. 12 radiant on a failed CON 17 save.

Hoping to get all of them with this.

Peri:

With Gerrard and Boko effectively blocking the doorway, Peri circles back behind them to get a clear view of the dead things through the door. He targets one of the glowing forms with a blast of eldritch force.

EB#1: 25 for 4

EB#2: 23 for 2

Then he eases back towards Qivys in order to leave a clear line of fire.

A silvery beam of pale light shines down upon the five corpses, all competing to get through the shattered, distorted doorway. They are engulfed in ghostly flames as they try to escape the searing radiant beam. (Saves below.)

Peri circles behind Gerrard and Boko, blasting the first, glowing figure (#6) with twin beams of crackling energy before retreating. (Both hit.)

DC17 CON Saves: Drowned Blade #2-#6: 9, 10, 13, 16, 11

Drowned Blade #6 attempts to Shove Gerrard out of the way. STR check: 19. (Gerrard: make a (blessed) contesting Athletics or Acrobatics check or be pushed backward 5 feet as the corpse enters the kitchen. If it fails, the thing retreats back out into the scullery with the others, outside of Boko's beam.)
Then it swings its rusted blade at Gerrard.
Rusted Longsword Attack vs. Gerrard:18.  Hit: 4 slashing damage (and Gerrard must succeed on a DC 12 Constitution saving throw...)

The others retreat further back into the scullery, outside of Boko's silvery beam, silently screaming in searing pain from the arcane light.
Thwack and Qivys (still concentrating on Bless) may act. 
Gerrard (still blessed) must make that save (or two) and then may act as well.
Note that anyone - friend or foe - entering the 10' cylinder of light right outside the kitchen door is subject to radiant damage upon failing a DC17 CON save as long as Boko maintains her spell. But those in the kitchen can still see the creatures, including some still lit up with Peri's spell.

The four drowned blades in the back, having retreated from the light, are dodgeing for their action. Disadvantage to hit all, except the fairie fire-affected ones.

Gerrard:

Attempting to stymie the corpse’s advance, Gerrard maneuvers adroitly with his paired weapons:

Acrobatics with Bless:  21

Qivys will launch another couple of bolts at the creatures, focusing on the glowie ones. 

1st: attack: 15
Damage, if hits: 6

2nd attack: 20
Damage, if hits: 3

"Well," Qivys says. "At least I got a good nap befor' my death." he laughs as he finishes his statement. 

Gerrard dodges the shove and holds his ground; the creature retreats to the north in defeat (Gerrard may get an opportunity attack vs. #6 with advantage and bless.)

Qivys fires a pair of bolts at #4. (Dodging; so advantage is negated. Fortunately, both still hit, if barely, as the sluggish zombie-like creatures seem to be quite easy to strike. It is looking in rather bad shape, as is #6.)

Thwack and Gerrard (blessed) may act next. (Gerrard may first have that AOO.)

Gerrard:

Locking blades with the drowned blade, Gerrard manages to use superior leverage to block the creature’s entry and shove it back with a snarl.  He attempts to get in a dagger strike as it moves away from the searing moonlight.

Dagger AOO:  Attack: 27 Damage: 12 plus Bless:  3

Thwack 

(Assuming Gerrard just finished off #6 as it was in rather bad shape)

Thwack grunts in satisfaction as the corpse crumples at Gerrards feet. Through the door he watches the rest of the hoard shuffle out of the magical light. He moves until he can see a corpse with Peri's colourful outline and let's loose 2 more mind daggers.

A - Attack: 21 Damage: 9

BA - Attack: 22 Damage: 6

Gerrard:

As the creature either retreats or falls at his feet, Gerrard stays partially blocking the doorway and focuses on defense, allowing his companions with ranged attacks to unload on the hapless undead.

Dodge