Since I'm nice, I'll give you a reroll, if you want it: 15

Ruin contemplates for a moment, then he makes his request.

"After this battle, a serious gap in my defense has been forcefully exposed," the robot says. "If it can be done, I wish that you could fortify my armor against magical weapons in addition to common blades."

"Ahh, a warrior at heart. Here, take this, it should protect from less...mundane threats" Stalhaer conjures a small pendant, in the symbol of a bat. It grants the requested effect as long as it is worn.

"Let the shadows of the night protect you. Now, is that everyone? I think so. Let us continue. Which among you slew the Runelords? Who dealt the killing blow?"

Kass looks around. "I think I finished off the last one - but everyone had a hand in it."

"It does not matter who helped. it only matters who struck the killing blow. So it was you, and who else?"

So you don't have to go back and look, Kass's celestial killed Divine, which would give Kass credit since it was a summoned spirit, Evelyn killed Arcane, and Kass killed the Primal.

"It does not matter who helped. it only matters who struck the killing blow. So it was you, and who else?"

So you don't have to go back and look, Kass's celestial killed Divine, which would give Kass credit since it was a summoned spirit, Evelyn killed Arcane, and Kass killed the Primal.

"Well, since it seems no one has come forward with who posseses the spark of Undermountain, I will have to take it" Stalhaer sneers, and begins to transform. His flesh melts away, his bones turning to dust, until all that remains is a large, humanoid shape made of blood. With a glip, and a glop, he lunges forward, ready to suck the life out of Kass!

This time, we'll use group initiative (hopefully it will help combat run smoother):

The Party goes first!

You may act in any order!

"Unbelievable," mutters Amari under her breath. "No respite yet..."

The cleric once again calls upon the divine radiance of her guiding bolt.

Ranged spell attack (cast at level 5):  32 to hit, causing 20 radiant damage

Amari, thoroughly annoyed at the amount of blood-shed that has been spilled today, sends forth a beam of divine energy, which strikes Stalhaer, causing a burst of congealed blood to splatter on the ground

Keep those turns coming! The rest of the party can GO!

Wripers, primed and with a full belly, lets out a vicious cry, his heart filled with both malice and the unquestionable urge to crush this new enemy! Worse yet, he's still a little hungryyyyy!!!

[Zealous Presence grants allies in 60 foot diameter advantage on attack rolls and saving throws, until Wripers' next turn.]

His first swing flies from below, a glancing blow with a heavy hand and strong retort. But it's the second strike from above that carries more promise. 

[1st to hit: 24, damage: 16]

[2nd to hit: 29, damage: 13]

"Flesh and blood creatures are so fickle," Ruin sighs. "First giving gifts, then attempting to destroy us."

Ruin uses his bonus action to cast spirit shroud (radiant damage).

Then he attacks twice, with advantage from Wrippers' Zealous Presence.

Greataxe vs. Stalhaer
Attack: 33; Damage: 21 (magical slashing) + 8 (radiant)
Attack: 37; Damage: 13 (magical slashing) + 8 (radiant)

Wripers and Ruin, both mighty warriors, surge forward in tandem. As Wripers let's forth a primal battle cry, the declarations of fury and death forecoming, he brings his hammer up from down below, as the weapon slams into oozing blood. With another grunt, he swings his weapon around, and crushes more of the foe. 

As Wripers knocks Stalhaer off his guard, Ruin comes in with a mechanical clank and no small amount of exasperation. With two powerful strikes, he summons an aura of blazing holy energy, which wreaths his blade in a glowing light. Both attacks land, cleaving away great glops of congealed blood, the substance something like jelly or jam.

Kass and Amelia! GO!

Amelia groans, "Oh come on, ya don't need to fight us!" She sighs, casting grease on Stalhaer's space, moving back to let the party and puddle do the trick while she charges up a blast in her rifle

(So thats a dex save from the creature)

Stalhaer's DEX save: 10

A puddle of oil and grease spreads across the floor (I assume it was positioned in such a way that it wouldn't hit Ruin and Wripers)

Stalhaer is now Prone

Kass, GO! (weapon attacks have advantage since he's prone)

(Sorry all, been out of town)

Kass recoils from the initial lunge, but years of practice enable him to draw his weapons defensively and reposition.  His companions do what they do best, and Kass takes every advantage he can get.  

Two attacks from the Vorpal scimitar at advantage: 
Attack 1: Attack: 18 Damage: 13
Attack 2: Attack: 19 Damage: 11

Bonus offhand attack from the rapier: Attack: 21 Damage: 9

With practised skill, Kass leaps forward, blades out, teeth bared at the oozing creature which lays upon the floor. With the first two strikes, both from his wickedly curved main-hand blade, he cuts away chunks of substance, and then comes in with his off-hand rapier, which digs right in to where Stalhaer's skull should be.

Stalhaer's Turn!

With a grunt, and then a wicked grin, the blob hisses up at the Party "Let's flip that around shall we? An assault of the mind for an assault of the mind" And then Stalhaer melts, becoming a puddle. A second later, his swirling body reforms behind Kass, and with a vicious lunge, slings two heavy tentacles of sapping blood at Kass's head

Tentacle Attack #1 VS. Kass: 20 to hit, 19 Bludgeoning plus 20 Necrotic damage

Tentacle Attack #2 VS. Kass: 30 to hit, 22 Bludgeoning plus 25 Necrotic damage

Whirling around, a third tentacle erupts from the blood man's forehead, and slithers through the air, striking at Amari. She must make a DC 22 CON save or:

Party, GO! 

Ruin makes a mechanical sound that is probably some kind of expression of exasperation. But it is his duty as a soldier to protect his comrades, and there is no time to be angry or tired.

He swings his axe at the disgusting mass of blood and flesh.

Greataxe vs. Stalhaer
Attack: 36; Damage: 20 (magical slashing) + 6 (radiant)
Attack: 35; Damage: 15 (magical slashing) + 8 (radiant)

(Note regarding spirit shroud: any creature hit with an attack that benefits from this spell cannot regenerate hit points until the beginning of Ruin's next turn)

(Kass' resistance to Necrotic damage saved him this round, but he's looking rough!)

Kass lurches, covered in blood, both the creature's and his own.  Snarling, he extends his finger and a sickly green beam of energy snakes out and wraps itself around the bloodbag.

(Casting Disintegrate at 8th level, DC19 Dex save or take 101 force damage)