"Barnacles can be sharp," warned Ola just in case, but still curious helped to best of her ability. Shape water making sort of "well" by freezing water around the sinkhole and then taking out as much water from inside as possible (at least 5 cubical ft, but can be repeated, if I am not mistaken, since effect is instantaneous).

"Sylyra, to be safe, tie a rope around your waste.  No telling if that sink hole could suck you down if anything shifts." Uragum shouts out. 

Zelra waits for the group at the door, leaning against its frame.

Arnald is anxious to head down the corridor but waits for Uragum to give the all clear.

Sylyra gives Uragum an impatient look, but does as suggested, tying one end of the rope around his waist and offering the other end to Ola. Once she has as much water scooped out as she can scoop out, the elf jumps down to check out the bottom and sides of the sink hole.

For Uragum:

The water slowly flows from the sinkhole to the frozen well created by Ola. After two feet of it have shifted, the lowered water reveals a human head, followed soon by another, both ravaged by their apparent undeath. The heads twist and turn to look at those that revealed them, before the pair of ghouls leap into action, claws tearing at the sides of the sinkhole as they move to attack.

Initiative:

Round 1 Order:
Players <=====
Ghouls

Placement:
Arnald and Zelra are at one end of the chamber by the door.
Ola, Sylyra, and Uragum are at the other end of the chamber by the entrance.
The ghouls are at the bottom of the sinkhole.
All combatants are within movement range of one another.

Ola made a short laugh, shaken her head: "I think this is the first time my silly useless sensitivity might had help! And I ignored it!" 

She restored her shimmering shield around herself but instead of rushing in on the slippery floor, begun with scooping some water, freezing it (or taking a piece of ice from the "well's" walls) and dropped that icicle (shaped to get inside) down the the sinkhole on the heads of the ghouls. 

Bonus action: Shield of Faith

Action: Shape water

(I am aware cantrip should not harm, but in this case the gravity should be the one hurting. 5 ft³ = 37.402600 gal or 141.57440 Liters or 141 kg. Might hurt :) )

Sylyra shifts his position around the sinkhole to get a good shot at the two climbing ghouls (as close to behind them as he can get). He begins waving his hand in an intricate maneuver, and a shard of ice quickly forms in front of the elf. With a final flick of his wrist, the shard is propelled toward the ghoul closest to the top.

Ice knife attack vs. the first ghoul: 10, damage: 2 piercing damage. Both ghouls need to roll DEX 14 or sustain 7 cold damage

"Yikes." Zelra takes out her shortbow, stepping a little closer to get a better look of the two creatures and shooting at one of their heads.

Action: Shortbow Attack - 15  Damage - 8

Hearing the commotion behind him, Arnald turns and is surprised to see the ghouls. He quickly grabs his bow and an arrow and looses it into the closest ghoul.

Attack: 22 Damage: 8

With the art and everyone starting to rush back.  Uragum's attention is directed to ghouls.  Following suit with Sylyra, he throws his own ice knife at them.

Ice knife to the least injured one. 

Attack: 17 Damage: 8

All within 5 feet must make a 13 Dex save or take 8 damage

OOC: No problem with Ola using Shape Water in that way, but it will take two actions; one to move the water above the ghoul, and a second action to freeze it before dropping it.

DM Screen:

Ghoul DEX Save: 6
Ghoul DEX Save: 12
Ghoul DEX Save: 22
Ghoul DEX Save: 19

Claws vs. Arnald: 15 Damage: 7
Claws vs. Uragum: 13 Damage: 6

Arnald CON Save: 20
Uragum CON Save: 8

The ghouls' arrival on scene is met with a hail of missiles in the form of arrows and ice. While Zelra's shot misses, Arnald's strikes a ghoul in the chest, moments before Sylyra's ice knife impacts with the wall and explodes, showering them with icy shrapnel even as Uragum's own ice knife explodes within a ghoul's arm, tearing the limb to pieces and ricocheting into its companion.

A shimmering field of energy surrounds Ola while she moves water into the air above the ghouls in preparation to freeze it. However, the broken and bloodied undead creatures scramble out of the sinkhole beneath, one clawing its way to each end of the room. Arnald and Uragum are the chosen targets, each feeling the claws of the surprisingly nimble creatures rake through their flesh, accompanied by an unpleasant burning sensation.

Unworldly sounds accompany long tongues that dangle from the creatures' mouths, free from jaws that no longer feel.

Round 2 Order:
Players <=====
Ghouls

Placement:
Arnald and Zelra are at one end of the chamber by the door.
Ola, Sylyra, and Uragum are at the other end of the chamber by the entrance.
One ghoul is engaged in melee combat with Arnald.
One ghoul is engaged in melee combat with Uragum.
All combatants are within movement range of one another.

Health:
Both ghouls are badly wounded.

Uragum is wounded.
Arnald is lightly wounded.
Sylyra is lightly wounded.

Ammunition:
Arnald has used 1 arrow.
Zelra has used 1 arrow.

Delta's face twists into a look of disgust as she sees the creatures crawl out of the hole, revealing their full bodies and letting out horrific screams. "That's just gross."

Action: Shortbow Attack - 17  Damage - 8  Sneak Attack - 5

With the ghoul being grasping, clawing and biting at him.  Uragum did what he could.  That is, he tried to ram the dagger into the creatures skull.

Attack: 20 Damage: 6

Sylyra grabs his staff with both hands, and once again, green flames dance across the weapon's surface. If Uragum doesn't dispatch the ghoul attacking him, Sylyra attempts to unanimate the reanimated dead. Otherwise, he goes for the one attacking Arnold.

Bonus: shillelagh

Action: shillelagh attack with quarterstaff: 25, damage: 6 magical bludgeoning damage

With a sickening thud that ripples the water, Uragum turns one ghoul's head into paste. The other falls before it can strike Arnald again, an arrow from Zelra planted firmly in the back of its skull.

OOC: Each character has received 80 XP for defeating the pair of ghouls.

Room Description:

The floor levels out in this room, which is circular and 40 feet in diameter. Despite the water that fills the room to a level up to your shins, it is easy to notice the sinkhole that has opened in the middle, though seeing the bottom is impossible due to the amount of sediment that has accumulated here. The sides of the sinkhole are gently sloped, but covered in a thick coat of slick algae.

The only other feature of this chamber is a narrow hole near the top of the left wall and an open door of beaten brass on the far wall. From here you can see a smear of dried blood upon the handle.

Sylyra, still looking at the sinkhole, asks, "Dare we investigate further?"

Arnald sighs, "Might as well."

Looking over at Zelra, "Thanks for the save. I was in a bit of a bind having my bow out with it that close."

Zelra shrugs and leans back against the door frame. "No problem, man. And yeah, might as well. We... well actually, you guys went through all the trouble to open it up. Might be some treasure down there."

"I'll remove the water," did not argue Ola, ready to help anyone willing to dive in the hole.