To be clear, you were attacking the bone figure, which is 'alive', rather than the flesh figure, which is 'dead'?

With a muttered proclaiment, Ruin swings his axe twice, the first strike, though solid, pale-ing in the absolute viciousness of the second, which cleaves through bone, causing more of the black substance to spill onto the ground.

Rest of the party, GO!

Yes, Ruin is trying to kill whatever is still moving.

Kass coughs up a bloody hairball, and casts Mass Cure Wounds at 6th level again, healing himself, Wripers, Amari, Amelia, Ruin, and Evelyn for 27 hp (rolled in the game log)

Healing magic ebbs and flows throughout the chamber, multiple different spell-casters manipulating the strands of life energy, an invisible web that covers all the world

Wripers, Amari, Amelia, GO!

Working in tandem with her summonned celestial, Amari attempts to strike her foe first...

• Guiding Bolt: 34 to hit, causing 31 radiant damage

She then commands the Spirtual Avenger to once again target the obsenity that is, or once was, Stalhaer...

• Avenger' radiant longbow, shot 1:  24 to hit, causing 16 radiant damage
• Avenger' radiant longbow, shot 2:  31 to hit, causing 16 radiant damage

With little issue, Wripers stands once more, following suit with Amari's Avenger to deal a serious strike of his own. He swings downward twice, as if to nail the creature into the earth like a tent spike.

He neither holds back nor misses, trying to shatter the earth along with this monster's dusty old bones!

[First Strike, To hit: 33, Damage: 28 (17{+11 Divine dmg})]

[Second Strike, To hit: 34 (Nat 20), Damage: 21 (crit)]

Amelia let's out a breath feeling the firebolt charge a bigger spell, firing off scorching ray at fifth level

31 to hit, and 7 fire

36 to hit, and 5 fire

22 to hit, and 9 fire

33 to hit, and 3 fire

32 to hit, and 8 fire

19 to hit, and 11 fire

Amari, who by now is sick and tired of all these blasphemous creatures, channels her righteous fury into the power of her Guiding Bolt. But, Stalhaer, whose bony form is small and agile, dodges at the last second, and the spell strikes a mere glancing blow. Her summoned angelic being however, has masterful skill and luck aplenty, sending forth two blazing arrows of light that leave the skeletal foe spinning.

But, as he recovers from the dodge, Wripers charges in, hammer blazing with divine energy. His first strike is strong, sending Stalhaer reeling. The first, however, pales in comparison to the ferocity of the second. As Stalhaer stumbles back, Wripers collects himself, then with a mighty roar, launches himself into the air, brining his hammer over his head in a downward arc which connects with the bone skull of his undead foe with a shattering "CRACK!" that resounds through the chamber. The face of Stalhaer, which is no longer a grinning visage, but a vicious frown, has spiderwebbed cracks running all along it from Wripers mighty strike

Then, as the furious waves of rage flow off Wripers in a terrifying display, Amelia's cannon finishes charging, and with a grunt and flick of a lever, sends forth six fiery projectiles, which all fuse into one at the last second, a twisting mass that forms one massive flaming arrow, which strikes the stunned Stalhaer right in the chest. With a burst of light, shards of bone fly everywhere, as Stalhaer explodes. Everyone, make a DC 18 DEX save or take 18 Piercing damage,

As you all recover, the dust and ash clear to reveal something lying where Stalhaer is: a small, fleshy lump. It' shape and size are almost akin to a fetus (or, to use pop culture, Voldemort from the Harry Potter movies before he was put in the cauldron. It also looks a bit like Gollum). The thing seems to be breathing heavily, the whole body expanding and contracting. It lets forth a piteous moan, high pitched and grating, and tries to speak, it's word faltering. Through the convulsions of its lungs, you can make out, the following fragments "I...defeated... Go...kill...please!" it lets forth another shuddering howl, and then something akin to a cackle, the pain and suffering evident in its every noise.

[Dex Save w/advantage (thank you, Danger Sense): 18 on the dot]

Dex save: 12

Dex save: 25

"What do you think? Put him out of his misery?" Kass asks, limping forward and brushing bone fragments off his tunic. 

Dex save: 16. But the automaton does have resistance to Piercing damage.

Ruin looks down at the disgusting little thing with pity...

"That would probably be the kindest thing we could do," the automaton says morosely, bringing his axe up in preparation to strike the killing blow. "It is a shame that he turned on us like he did. Hopefully now he can rest in peace..."

Ruin brigs down the axe, a single powerful strike on the prone target, with thunderous energy crackling on the blade and still empowered with radiant energy from ancient spirits.

Attack: 36; Damage: 19 (magical slashing) + 9 (thunder) + 8 (radiant)

Ruin raises his axe, and as the creature piteously moans, brings it down in a decisive strike. With this final strike, the final one in a long line of ones that shed blood, carved flesh, and shattered bone. 

As the blade strikes into the horrible creature, cleaving it in half, a surge fills the chamber. A surge of something. Crackling green energy issues forth from the broken form, electric sparks flying everywhere. As lime death fills your vision, you... and then it stops. For a split second you stand there. You feel...woah. Oh. Your vision blurs, and you fall to your knees. The room seems to shake, and as your eyes close into darkness, you feel something slipping away.

Everyone make a CON save

Con save: 18 (also rolled a second time on the off-chance that whatever's happening is poison-related, but got a 15 so it doesn't really matter)

[Con Save: 19]

Con save: 16 - but resistant to necrotic damage and immune to disease, if that helps.

Can I have CON saves from Amari and Amelia?

Amari's CON save: 29

Botting Amelia's CON save in the game log: 28.

And Kass's resistance to necrotic damage gave them advantage on this save, but I rolled a four in the game log.

You awaken, sprawled across the floor. The room is dark, the torches having long gone out. The only illumination is the faint glow of your magic items and your magic selves (in the case of the glowing runes inscribed on Ruin's body). You feel as though you have woken up from hours of sleep. You all feel refreshed, though Kass and Amari both feel like something is missing, though they don't quite know what.

Everyone gains the benefits of a long rest. In addition, Kass and Amari both lose the blessing that Stalhaer gave them. What do you do?