Ember flies up for a better angle while yelling, "Cube!"   Touching thumbs together and spreading his hands wide, a thin sheet of flames shoots forth from his outstretched fingertips

Burning Hands      11 pts of Fire damage 

As soon as Razzec hears Ember yell, he immediately swings his Glaive at the Cube and upon hitting it activates his Fire Rune (the Cube must beat a DC 16 Str Check or be restrained for 1 minute) and Fiery Shackles wrap around the cube.  They are at least shackled until the end of their end.

Glaive+1 - Attack Roll 20 (not nat 20), Damage 8 + 7 (Fire Rune activation dmg)

Glaive+1 - Attack Roll 16, Damage 13

Action Surge - Glaive+1 - Attack Roll 14, Damage 14

If it is restrained, it'll take 2D6 dmg at the start of its turns.  It'll continue to take that for 1 minute or if it manages to beat the DC at the End of its turn.

DEX save 5

STR save 21

The cube is Restrained.

Razzec yells at the others, "The cube is restrained for now.  Get Grey out of the pit or try knocking the undead back into the pit or into the cube."

Quote from Shasher >>

Grey, confident in the others abilities to handle the undead for a bit, will focus on searching the pit for any keys.

He will call on Moradin's guidance, knowing the key's importance.

Perception: 24

I'm assuming thats his action, but if not or combat ends, he will try to climb out... Athletics 20 to get out. [LMK if you want something different]

 

Before I leave the pit, did I see anything besides the cube with this?

Quote from Shasher >>

Quote from Shasher >>

Grey, confident in the others abilities to handle the undead for a bit, will focus on searching the pit for any keys.

He will call on Moradin's guidance, knowing the key's importance.

Perception: 24

I'm assuming thats his action, but if not or combat ends, he will try to climb out... Athletics 20 to get out. [LMK if you want something differen

 

Before I leave the pit, did I see anything besides the cube with this?

You notice that there is a key piece inside of the cube right before it splorches into the pit.

[I'm assuming it's my turn as the cube just moved, if not we can save this for my turn or i'll repost if it makes sense]

Does the cube get pierced by the spikes below?

 I believe Grey is 5 feet from the top of the pit, so he will climb out of the pit and yell out, "There is a key piece we need, inside the cube!"

 He will throw a Sacred Flame at the cube. DC 16 Dex save or 10 Radiant.

Ember backs up Grey with a spray of three fiery rays at the cube Scorching Ray 

to Hit:  15     Dmg:  7 fire
to Hit:  22     Dmg:  6 fire
to Hit:  21     Dmg:  9 fire

"Once it is ashes, we can sort things out!"

Nico, seeing the cube move and fall, bounced up and down a bit as he grit his teeth and tried to summon some extra courage. But he just couldn't bring himself to dive into the pit with that... thing.

"Hit it again!" he yelled with resigned encouragement to his companions.

He'll give Bardic Inspiration (1d8) to the next person making an attack roll to hit the ooze.

You eventually burn it down from above.

Now you have a 20 foot deep spiked pit, halfway filled with acidic goop, with the key piece on the bottom.

YAY win!

"Any chance a mage hand could pull the piece (and anything else in the goo soup) out of the pit?" Ember asked with some trepidation. 
"Worse comes to worse, I can try to create some water to maybe dilute the acid?"  Ember shrugs as he flits to the ground beyond the pit to examine the hall space where the Cube came from.   

Passive Perception: 16

It is a room shaped like a hallway. It is also very clean!

It is possible to reach the key piece with a Mage Hand.

Ember sighs after looking about at the very clean hallway.  "Darn cubes," he muttered.    Flying back to the 'initial doorway side of the pit, Ember will land and pull out their water skin.  Dribbling a few drops on their hand, Ember will speak the arcane words, "aqua multa apparent" and make odd spinning motions with their hands.

Create or Destroy Water   

Seemingly from the air itself, a rain falls into the pit; dumping 30 gallons of water (cast at 3rd level) which swirls and mixes with the gelatinous goo below.  Ember is hoping it is enough water to at least dilute, if not neutralize, the goo.

Anton leans forward and peers down into the pit, looking into the soupy mess below. "Someone is going to have to down in there to get the key, aren't they?"  He starts uncoiling a rope from his pack to help lower and raise someone into the pit. 

Grey says, "I'm not going to lie, that trap kicked my ass, I'll need to do some self healing before we move on. If someone wants to climb down to  get the key, iI will cast Protection from Energy Acid on you."

"I've got it," said Nico, relieved that the thing was dead at least.

As he felt Grey's protective magic cover him, he nodded to the elf, then tied the end of the rope around himself securely before moving to the edge of the pit. Once everyone had positioned themselves to support his weight, he stepped over the edge and began his slow descent.

With the wierd way D&D rules work, if you go down tied to a rope, the acid will eat the rope. If you go down with a coil of rope on your person it will be fine. You can have the party throw down a rope when you come up also.

In any case, you take 6 Acid damage going down, 9 Acid damage coming up, and need to make a DC 10 Athletics/Acrobatics check to throw the rope to your friends, or catch a rope if they throw it down to you. If either fails, you take another 5 Acid damage.

Halve the damage due to Protection from Energy.

(( Oh! He'll just go down and then catch the rope the others throw down. Acrobatics Check: 6... total of 8 damage taken (plus 4 if the check fails.) ))

The bard hisses and yelps while down in the pit, but eventually retrieves the key and secures it before getting pulled up.

Once up and out of the pit, he lays down on the dungeon floor for a moment with his eyes closed. "Was... not.... fun...."