(OOC: I have adjusted Vidruth’s hand)

Nosam plants his feet, leans his shield into the door, and with Berry’s help, presses his weight into the wooden surface, which bows inward for a moment before, with a groan and a cracking sound when the very bottom of the door, which had swollen to meet the floor, splinters, old dry shards dragging on the shattered old tile floor, the door explodes into the cottage. A resulting poof of dust and ash from the decades old undisturbed room makes Nosam and Berry squint, their eyes reddening in irritation and causing both to cough.

As the dust and ash swirl in the half light, Perrin’s spell combined with the diffused sunlight beaming in from above throw crazy shadows into the room. Silence settles into this space, which, it seems, used to be the common room of a tavern.

But the silence doesn’t last for long. A slouching stirring draws Nosam’s attention to the corner of the room directly opposite him where an old pile of ash-covered laundry, seemingly, lies falling off a broken chair. But no, it is not merely laundry. A ragged head, one ear missing, toothless, lips black and oozing, covered in ash, straightens, peers with sightless eyes at Nosam, ragged sock-covered feet and legs uncurl and touch the floor, and slowly, a zombie stands with a deathly groan and takes a few steps toward Nosam.

I am setting initiative as follows:

Nosam and Berry may act now in any order.

The Zombie will then act.

Then the remaining characters can act in any order.

You are now situated like so:

Nosam & Berry, please post Round 1 actions in any order.

Berry almost fell inside the room when the door finally gave in. In the last moment he could catch himself and just in time to see how the mound of clothes, meat and bones assembled itself to shaky zombie. He presses himself around Nosam and closes in on the undead and cuts it with his scimitar.

Nosam gathers his feet under him and immediately charges toward the zombie. He swings his hammer almost in unison with Berry's attack. "Come on in here and see what's to the left."

Movement: move to R2, next to Berry
Action: Attack Z1 with hammer. Hit- 18 (13+5), Damage- 9 (6+3)
Bonus Action: None
Free Action: Talking

Round 1 Results (partial)

Berry flits past Nosam and slashes the shambling undead creature straight through its belly, opening it up. But the fairy is suddenly enveloped by a cloud of noxious vapor and microbe-filled ash particles that poof out from the gaping wound, while the zombie seems hardly to notice that its stomach is half gone. 

(Berry: Any living creature within 5 feet of the zombie must succeed on a DC 10 Constitution saving throw or gain disadvantage on attack rolls, saving throws, and ability checks for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on it early with a successful save.)

Nosam’s attack slams into Zombie 1’s head, sending its jaw flying through the air to smash against the wall. But its resulting unholy half face is even more macabre when it attacks.

(N.B.: there is no ash puff on the second attack, only the first time the zombie takes damage. Also, it’s common knowledge among adventuring types that zombies can be destroyed with regular weapons, but they don’t mind/notice losing limbs et viscera in the process.)

The wounded zombie’s fetid, wiry hands grab Nosam’s hammer arm immediately following the dwarf’s attack and slam it against the edge of the bar with necrotic strength.

Now two more Ash Zombies stir and rise with guttural groans, one pushing out from under the ash and collapsed furniture in the southeast corner, and another which had been stretched out on its face in the ashen dust behind the bar. They move to S3 and R1, respectively, the first attacking Berry, the second attacking Nosam, pounding them with sooty fists.

Ash Zombie 1 v Nosam: 11 to hit / 6 HP bludgeoning

Ash Zombie 2 v Berry: 7 to hit / 2 HP bludgeoning

Ash Zombie 3 v Nosam: 4 to hit / 3 HP bludgeoning

(Berry: please make the saving throw above.)

Marten, Perrin, and Vidruth may now act in any order.

Marten: Round 1

"I reckon we found th' zombies!", Marten drawls.  Then he exhales and tries to relax and focus on his aim.  He has a clear shot  on the zombie at S3, and he tries to fire before anyone else runs through the door or gets in the way.  As soon as he fires, Marten draws another bolt to reload his crossbow, not taking his eyes off of the fight inside the building.

Movement: None.  Remain at P6.
Action: Attack the zombie at S3, Crossbow, Light (80/320): 16, Damage: 21
  (Advantage from Stead Aim.  -5 to hit for +10 damage bonus from Sharpshooter feat.)
Bonus Action: Steady Aim, advantage on the attack as long as Marten does not move / speed = 0' during his turn
Free Action: Talking, reloading crossbow

Berry’s eyes go wide when the gash in the zombie’s belly releases a smoke bomb. Hurriedly, he closes his mouth, cheeks puffed to bursting, and regrets that he never got along to learn from a camel how to close his nose. (Constitution save, DC 10: 10)

vidruth snarls something about inefficiency and just wanting a damn sword, then with a gesture sends the hand forward to Q1 and follows somewhat, stopping at Q6

Perrin nearly laughed at Marten's wry observation, but adrenaline caught the chuckle in his throat. The unnatural illumination and the clouds of dust the creatures had kicked up made it hard to see, and the halfling found himself squinting to take aim at the undead abominations. Seeing one, he envisions a pair of dice rolling in its eye sockets, the pips coming up snake eyes in his imagination. Your luck's about to run out a second time, my  putrid friend,  the luckbinder thought, as subtle wisps of fate and misfortune began to bind themselves around the zombie. He then fired a bolt of arcane energy at one of the wretches. "Y'know my cousin is a druid, and he usually just chats with squirrels and cultivates the best pipeweed in the valley. Fewer killer plants and zombies. Are we sure this Reidoth will welcome visitors?"

Round 1 results (final)

Marten’s crossbow bolt flashes through the doorway to strike zombie 3 through its ear and continue out the softened bone of the other side, causing its head to explode in a cloud of ashen diseased zombie brains. Unfortunately, the zombie brains envelop both Berry and Nosam in gruesome fallout while the creature drops to the floor, motionless. 

(Berry & Nosam: Any living creature within 5 feet of the zombie must succeed on a DC 10 Constitution saving throw or gain disadvantage on attack rolls, saving throws, and ability checks for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on it early with a successful save.)

I picture Vidruth’s hand descending underground and then emerging through the ruined tavern’s floor at Q1 as the chair tips over sideways to make room for it.

The zombie at R1 has cover from the bar and almost everyone in the room. But its shambling movements are too slow for Perrin, who manages to blast it with his arcane powers, making an 8-inch hole through its chest, causing another cloud of putrid ash to poof from its now-revealed charcoal lungs to envelop Nosam in another layer of putrefaction. (Nosam must roll another save vs this effect.). The walking corpse shows little interest in the damage and remains focused on the sturdy dwarf in front of him.

You are now situated like so:

Round 2

Nosam and Berry may now take actions in any (either) order.

(Reminder: Nosam must make 2 CON10 saves before taking his turn with the results of failure described above. Also,  Z1 has 4HP, and Z2 has 5. They are AC8. You may narrate their demise if you cause it.)

This time Berry was prepared when the ash zombie’s head exploded from Marten’s bolt and avoided it easily, it may have helped that he’s an entire head smaller and the cloud was way above. As he already ducked below the cloud of zombie brain, he arced his scimitar horizontally across another zombies lower torso and caused its insides to spill out (, ostensibly,) in another ash cloud while the zombie returns to his previous form, a heap of clothes, bones and rotten flesh. Berry had no problems to avoid the ash cloud at all this time but the room starts fill up dangerously quick with the dark smog.

2 CON throws: 24 (19+5) and 17 (12+5)

"Gah, the smell." Nosam coughs and hacks toward number 2 as Berry deals with the other. Looking toward the others he yells, "We need out of this small space!!" before swinging furiously just below the neck of Z2. He hopes to do enough damage without causing another cloud eruption. 

Movement: move to S1
Action: Attack Z2 with hammer. Hit- 20 (15+5), Damage- 8 (5+3)
Bonus Action: None
Free Action: Talking

All three zombies de-animate and thump to the floor like the lifeless corpses they should have been all along.

Marten, Perrin, and Vidruth may now act in any order.

Marten: Round 2

Marten hears Nosam and see them gasping.  He says, "Any more in th' building?  You could draw 'em out, 'n' hold 'em at th' door for us to kill."  He moves over to the window and attempts to open it from the outside.  That would let more air into the dusty room, and it would give him a good spot to cover Berry and Nosam.

Movement: Move to the S5, outside the building but next to the window.
Action: Attempt to open the window. (STR check: 12)
Bonus Action: Cunning Action, Hide - Stealth 11
Free Action: Talking

Quote from Ben_Evolent >>

Marten: Round 2

Marten hears Nosam and see them gasping.  He says, "Any more in th' building?  You could draw 'em out, 'n' hold 'em at th' door for us to kill."  He moves over to the window and attempts to open it from the outside.  That would let more air into the dusty room, and it would give him a good spot to cover Berry and Nosam.
Bonus Action: Cunning Action, Hide - Stealth 23

I should have mentioned that after Marten forces the shutters open, he crouches down to hide from anything inside the room until Berry and Nosam find more zombies or twig blights or any other new target for him.

vidruth frowns at the door "if someone could open that, ill grab the first one through"

action: prepare to attack the first hostile through the door with the hand

Seeing the zombies fall, Perrin breathes a sigh of relief. Not too deep, though, given the smell Nosam described. He positions himself so as to gain a clear view of the door, and makes ready to fire on any hostile beast that might burst through the door. The warlock takes a quick look around his immediate environs, checking if their activities had roused any other dangers. He nods to his comrades, and prepares alongside Marten and Vidruth.

Round 2 (final)

Marten: Move to the S5, outside the building but next to the window / open the window. (STR check: 12) / crouches down to hide - Stealth 23

Vidruth: prepare to attack the first hostile through the door with the hand

Perrin: positions himself so as to gain a clear view of the door, and makes ready to fire on any hostile beast that might burst through the door. The warlock takes a quick look around his immediate environs, checking if their activities had roused any other dangers.

Marten succeeds in breaking the rusted latch on the old storm shutters and pulls them open then ducks to hide. 

Vidruth and Perrin ready spells. Perrin does not see or hear any other creatures in the vicinity.

Round 3 (partial)

Berry (bot): Opens the door to the next room (interact with object). Nosam now sees into the other half of the cottage: a kitchen. A fine ash blended with dust covers the floor of this room too. There are shelves along the visible walls, and a kitchen work table with rusted old cleavers and other kitchen tools in view. Nothing moves.

You are now situated like so:

Nosam and Berry may now post for Round 3.

Berry peeks inside the room that he just now opened and spots a back door. „Marten, there’s a back door in the next room that leads to the other side! If you want to stay outside but observe the other room, check it out.” Then he waits for the other to respond or act but is ready to dodge anything coming at him.

Nosam moves toward the door and quietly tells the others before charging in, "I'll head in and start swingin' when needed; hah, don't go hittin' me in my backside." 

Round 3 (cont’d.)

Nosam: move to N3 (or until run in with a target) / Attack (if needed) first available target with hammer. Hit- 12 (7+5), Damage- 7 (4+3)

Berry: dodge

As Berry takes a defensive posture by the door, Nosam rushes into the kitchen. At first, nothing moves. But then, in the southeastern corner, a pile of old mildewed sacks shifts, and from beneath them, a short, stocky humanoid, covered in ash and soot – another ash zombie – groans, rising to its feet, and brandishing a cleaver, lunges toward Nosam.

Nosam’s readied attack is triggered, and the zombie takes 7HP damage.

You are now positioned like so:

Marten, Perrin, and Vidruth may now act in any order.