2014 edition.
So my warforged player lost his arm when he died to a treasure mimic (I believe there is still a thread here about it) and now they went to the duergar foundry where because of some plot related stuff the head blacksmith decided to not only fix his arm but to upgrade it.

I wanted to share the item I made as I want feedback and general feel of how it would feel if you were to use it as a player. Its important to note that my campaign is NOT running 6 combats per long rest (who does, that's insane!), its in its absolute maximum 2. My group enjoys narration and rp much more.

I also changed how the vulnerability worked so instead of multiplying the damage by x2 it is instead x1.5

It's also semi important to note that the character gave away the Flame tongue longsword for the cost of repairing and upgrading his arm. (He was willing to part it simply to fix his arm)

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So, without further ado, here is my Magmatic Crystal Arm:

Wondrous item (prosthetic limb), rare (requires attunement by the creature it is attached to)

Forged by duergar smith, this prosthetic arm is powered by a magical magmatic crystal. The crystal draws energy from the place of its creation through an unbreakable connection. Although this connection is inexhaustible, it cannot continuously sustain the crystal's maximum output.

The crystal naturally regains its power over the course of 1 minute. However, in the heat of battle it may recover more quickly.

The hand is always pleasantly warm to the touch.

Core Ignition (Recharge 5–6).

As an action, or in place of one of your attacks as part of the Attack action, you ignite the crystal's core. Until the end of your next turn:

Your weapon attacks deal an additional 2d6 fire damage.
You gain resistance to fire damage as well as cold vulnerability.
The arm sheds bright light in a 40-foot radius and dim light for an additional 40 feet.

After using this feature, it becomes expended. At the start of each of your turns, roll a d6. On a 5 or 6, the crystal regains its charge.

Sacrificial Recharge

If the crystal has not regained its charge, a creature attuned to the arm can force the crystal to recharge by sacrificing a portion of its own vitality. The creature gains one level of exhaustion, and the crystal immediately regains its charge.

Fire Bolt

The arm can project a bolt of magical flame.

You can cast Fire Bolt (2d10 fire damage), using Dexterity as your spellcasting ability for the attack roll as well as adding your Constitution modifier to damage.

Edit: The arm itself doesn't need to be attuned to in order to function, but every ability that the crystal provides does.

I am dming a 5th edition (2014) dark fantasy game where I want there to be stakes.

The party currently is traveling through the underdark while being lvl 6. During their travel through a giant underground sea they encountered an isle and a lot of tossed around, like rubbish, coin. Then they same even more and more until they saw a giant hill made of treasure and gold.

They immediately themselves said that it was the most obvious trap there is (they were already roughed up from all the other dangers such as drow, pirates and fish...)
So what did they do then? That's right, they approached the hill of gold, four of them sat there and waited for the wizard to finish a ritual just 5 ft from the hill of detect magic.

The others... they split to search the other areas of an isle in search of a mansion (they mistook this isle for the one they actually wanted to traveled to)
So... a few minutes later it attacked seeing the opportunity, almost downing the wizard which got off easily after that. It then prioritized a damage dealing fighter and grabbed it with the tentacle. Then comes a wizard's turn and he fires a fireball but because of narrative reasons he wasnt able to aim it properly, damaging everyone and downing the fighter.

Then came the monster's turn and it ate through the fighter to ensure its prey is eliminated (it is quite intelligent after all) then came the fighter's turn and even with the daily inspiration that I give them he still fails the death save and dies.

Then the party kills the mimic almost losing another member and they want to resurrect the fighter obviously. The problem being the fact that they are hobos - they didnt have diamonds. So they asked if the mountain of treasure would have it. I decided that its possible, but not guaranteed. so the fighter rolled to see if it was so. And he succeeded, diamond is somewhere among the giant pile of coins and other items.

NOW, my question is as follows: How should I dm the part where they try to find the diamond in 54 second (9 rounds, 9th being the last one when they can res him) fairly and entertainingly?
I want it to be tense but not impossible. There are 4 of them + 2 npcs, one of the npcs is down and bleeding to death.
Also if you have any other feedback or opinion I would love to hear it, even if its negative, I am no baby.