@Bertolt and @Talion
If you are both intent on using mage hand your rolls can be made with advantage [i don't see why not in this case] if you are both trying to get the sword.  maybe one can hold down and one pull?  anyways, feel free to roll either STR or DEX (not both) with advantage!

[don't forget to tell me what else you want to do - right now it seems like you're all on the stairs, scooby doo style, watching the bard and thief play with their invisible hands!]

Talion nods to Bertolt, as they both start to work on the sword. When they get it free, hopefully without collapsing the armour this time, Talion brings the sword back to examine more closely. "Big sword, anyone?" he asks, and if there are no takers, he sends it unceremoniously clattering down the stairwell, then shrugs, and turns back to the room. "

Dex (adv): 16

When the noises from the clattering sword abate, Talion gestures to Ervan to continue, helping him to check the room for traps. If it is clear, he investigates for hidden doors, compartments or hiding places, before finally moving to check the (collapsed?) armour.

Investigation: 20

Bertolt nods back and slowly moves the hand to pull at the armor a bit, attempting to be quiet but not with mich faith after what occured the last time.

Dex: 23

Talion and Bertolt expertly extract the greatsword from the armor's gauntlets - much quieter and much more successful than last time.
As you move into the room and get close to the armor to look around, you hear a creaking sound.  You can't place it until it's a bit too late - the armor is moving!  You look at it's terrifying visage and note two red dots where eyes should be...

Talion and Erven, you turn to face the threat but it has the upper hand - and surprises all of you!

Roll for initiative!

Initiative 
AA - 5 
? - 17 
?? - 12 

The animated armor groans, "Intruders," and swipes at Erven, the first one figure it sees:  Attack: 12 Damage:  4  . TO HIT - 21, DAMAGE - 3  (IT'S A HIT)

It makes another attack, again at Erven:  Attack: 21 Damage: 4   A MISS!

Edit - Messed it up again.  First attack was 21 for 3 damage. Second one was 13.  First hit, second missed.  Armor's initiative was 8.  - Sorry.  (? INITIATIVE = 21, 7)

MAP 

Erven initiative :  22

Initiative: 5

Talion initiative: 15

(Hoping that 22 is the high initiative, will begin)

Erven moves to the left (to the south and west) to try to encircle the animated armor.  He strikes with his short sword, in a flanking maneuver.  

Attack: 19 Damage: 7

(( Assuming 15 will also go before the Armour's next attack, and that Ervan has moved to flank the armour ))

Talion glances over his shoulder, looking to retreat down the stairs and behind Onyx, but seeing Ervan attack, he sighs and reluctantly decides to stand his ground with the crazy elf, at least for the moment.

(( If hex is running, then: ))

Bonus action transfer Hex to Armour.

Attack 1: 26 Damage: 7 (piercing) + 2 (necrotic) if Hex still running.

(( If hex is NOT running, bonus action, second attack: ))

Attack 2: 12 Damage: 1 (piercing) + 1 (necrotic) if Hex still running.

Add 2 bludgeoning to the first attack that succeeds.

Initiative Order (may change once Bert and Onyx roll)

 AA -

 Erven's short sword strikes the animated armor's plates and streaks down the side, leaving a scratch but little else - a miss!

? - holds action

Talion's dual attack with his daggers is more effective, the first attack tearing a large hole in the cuirass.  A bit of foul smoke seeps forth from this hole and you notice two glowing yellow eyes from within!  A big hit!

? will activate after the armor sustains a hit and will half slither half fly out of the tear in the cuirass and move above Talion in B2.  You can see this is some sort of flying snake!  It will strike at Talion and then move away to C4:  Attack: 15 Damage: 1 if a hit, damage type is piercing and if hit also take 7 poison damage!

AA - the armor reels from the hole in its chest, and moves to strike against Talion with a swing of its gauntlet: Attack: 7 Damage: 6 
It will then backhand swing against Erven, who it sees scurrying behind it:  Attack: 6 Damage: 3 
The armor screeches, which sounds like metal fingernails scratching a metal wall over and over again - not super pleasant.

?? - hold action....

Everyone else feel free to take your turns.  Onyx and Bertolt - you can stay where you are or make a roll.

Bertolt Inititiative: 16

Bertolt will ready his hand crossbow, aiming towards the armor before he s the trigger to let the bolt fly. Afterwards he will looks towards Oynx. "Show them your strength and why it was a mistake to challenge is!" He says to inspire his companion.

Crossbow: Attack: 8 Damage: 5

Sho softly gasps, his hand reaching back and pulls out the scroll they found in the secret room earlier, his fingers darted to tear open the seal and unfold it. The scroll records the component of the spell in exquisite and sophisticated writing, his eyes locked on the words on it as he recited the incantation, each word ringing true, emitting a faint glow on the scroll until the last word fell and the scroll turned into a sacred light that dissipated from his hands.

Bless: on Erven, Talion and Onyx

With a fair amount of time and the assurance of Erven, Onyx gradually emerges from his fear. Still, he is slow to move, and isn't where he needs to be when his friends are attacked. Still, he drags himself to the scene and tries to get into the fray.

Initiative: 18

On his turn:

Onyx sees his battered friends suffer for his overconfidence.

"Break off and get behind me," he shouts to the others as he comes up the stairs. He moves as close as he can to the armor and makes a strike with his longsword.

Attack: 13 Damage: 6

Erven has moved to flanking position and attacks the animated armor suit again, feeling the help of bless.

Attack: 26 Damage: 7

Plus to hit from bless :  1

Initiative

 @Onyx - you bull your way through Bertolt and come face to face with the powerful foe.  You take a wild slash with your longsword, but it's a glancing blow that draws a line along the bottom part of the cuirass.  The helmet swivels in your direction... - a miss!

@Bertolt - from the relative safety of the stairwell, you pop up and fire at the animated monstrosity, finding a clear seam in the line of sight.  The bolt hits home with a "thunk!"  The helmet looks down at the bolt.  You smile and watch as another set of yellow eyes gleam from within the tear caused by Talion's raging attack - another flying snake! (it's a hit!).  You collect yourself and call out to inspire Onyx Bardic Inspiration

@Sho - your eyes widen at the scene and you quickly reach in and grab the desired scroll from your pouch.  You quickly break the seal and activate the spell.  Erven, Talion, Onyx you enjoy the benefits of Bless unless Sho loses concentration...

S2 - the snake awakens on the hit from Bertolt and emerges from the visor of the animated armor's helmet.  It hisses at what it sees and launches into the air and flies to the ceiling.  Then it will dive and attack Erven before flying to the ceiling at B4:   Attack: 11 Damage: 1 and if it's a hit, also take 7 poison damage.

End of Round 1
MAP 

...As Erven pulls back to slash the giant metal thing, he feels a sting in the back of his neck.  Turning around, he sees the yellow eyes of the flying snake looking at him.  It opens its mouth, whispering familiar words,  "You can't do this alone" and a puff of yellow smoke billows forth. You close your eyes and cough but the inhalation of the smoke overwhelms you and you slump to the floor, unconscious...

FS1 keys in on the newcomer to the party, Onyx, and hisses as it moves to attack:  Attack: 9 Damage: 1 and if it's a hit, take 6 poison damage.
* the snake flies back from you and peers at you intently.  It seems to stare at you longer than is reasonable before hissing angrily and going back up the ceiling at C4.

Group 2 is up - Onyx, Bertolt, Talion! 

Onyx scowls only briefly at the flying snake as it fails to pierce his armor. He was more concerned with their animated nesting structure. If he didn't get to the source, he feared they would be overwhelmed.

Attacking the armor.

Attack: 15 Damage: 5

Talion looks at the weapons in his hands: a dagger and a meat cleaver. Then he looks at the flying snakes: Where once his weapons might have seemed ample, his recent experience suggests that they are inadequate to the task.  He's tall, but not tall enough. "Someone...deal with those snakes!" he shouts.

He's been here before -- facing enemies beyond his reach -- but where once he was alone, he now sees allies around him. Friends even. He's also still young enough to want to believe in heroes. With a resigned glance at the relative safety of the stairs, he grips the dagger and the meat cleaver in each hand, and salutes Onyx and the wounded Sho before he attacks. If not me, then who? 

Attack 1: 12 Damage: 5 (piercing)

Attack 2: 20 Damage: 6 (piercing)

 +2 Bludgeon if either if the above connect.

Edit: because of the weirdness of the dice roller, the first roll is not a critiical. Total damage is 4 for the first, 4 for the second, and 2 = 10

Edit2: If the armour is not still standing before or after one of the hits, then the rest are directed to one of the flying snakes...

Edit 3; if Athletics required (plain jump): 9, but if Acrobatics (using the stairwell wall or other features for acrobatic endeavours), add +2 more to the roll.

Bertolt will appear once more from his hiding spot down the step, looking at the situation before cursing and reloading his crossbow. He will attempt to aim for one of the snakes now that more seem to be appearing. "It may be wise to consider a tactical withdrawal. I have a spell that could help but you all would be hit."

Bertolt will aim for S 2

Crossbow: Attack: 23 Damage: 7

Erven thought he was going to attack the armor but as the poisonous snake emerges and strikes him, the initial sting is not so bad, but the poison enters his body and he gasps, sinking to the floor, now unconscious..

(at 0 HP)