Rizo also keeps his staff charged with Druidic energy (Shillelagh)

And in case Moni had advantage on her attack roll: 15

Minty Initiative  18

Summing up Initiative: 

20 - Lair action (if you want to do that)
18 - Spark and Minty
12 - Rizo and Moni
9 - Super-powered Chuul
6 - Mummy Lord (plus three legendary actions to be had at the end of other participants' turns)

The mummy lord uses its lair action to learn the location of nearby living creatures.

Spark readies a Magic Stone to launch at the mummy lord if it emerges from the pyramid.

It is Minty's turn.

Minty looks for a place to hide, loads a magic bolt into his crossbow and holds his action to fire at the mummy should it emerge from the pyramid.

Stealth:  23

The mummy lord doesn't use any legendary actions... yet.

It is Rizo's turn.

Rizo remains behind his waterfall readying a Fire Bolt for when the Mummy emerges. Moni stops on her way out of the pyramid to take another shot at the mummy and then moves to the chuul that waits outside.

Attack: 24 Damage: 16 fire damage (already doubled)

Moni's fire damages the mummy lord.  Enraged, it turns into a whirlwind of sand, moving to the pyramid's exit.  The party is waiting for it.

Spark's readied Magic Stone: Attack: 13 Damage: Unable to parse dice roll.

Rizo’s Fire Bolt: Attack: 27 Damage: 56 doubled fire damage

As the mummy steps into view, Minty lets his bolt fly.

Attack: 22 Damage: Unable to parse dice roll. 

Elven Accuracy die: 17 

Thank goodness for Elven accuracy.

That’s one unlucky mummy lord…

(Did Minty use a magical bolt?)

(Rizo was casting Magic Weapon on his heavy crossbow which is concentration and lasts one hour. I believe it is still active. Additionally, Minty loaded a magic bolt in his crossbow [#427], I'm not sure if that stacks, though, but it might.)

I was not sure if the spell was still active. If it is Minty would not use the magic bolt. The rolls assumed a +1. If the magic bolt was used it would be one more point of damage. 

Spark's attack misses the Mummy Lord, but Rizo and Minty hit and damage it severely.  It is currently at 3 HP.

The chuul tries to grab the mummy lord with its claws.

Attack: 25

Attack: 29

Although none of the fruits made the chuul's attacks magical, it is now holding the mummy lord with both claws.  The mummy lord uses its last legendary action this turn to target the chuul with a Dreadful Glare, trying to free itself.

Chuul's Wisdom save: 2

The chuul is paralyzed by the mummy's gaze.

Seeing that the most dangerous threat came from the waterfall, and knowing Rizo's approximate location, the mummy lord sends an Insect Plague to the waterfall.  Rizo must make a DC 17 Constitution save, taking 14 damage on a failure or half of that on a success.

The mummy lord retreats 20 feet into its pyramid.  It uses its Lair Action to once again learn the locations of nearby living creatures.

One of those creatures happens to be Spark, who casts Expeditious Retreat to chase the mummy lord to the pyramid's entrance.  She launches her remaining two Magic Stones at it.

Attack: 25 Damage: 9

Attack: 24 Damage: 11

Spark's Magic Stone finishes off the mummy lord.

Each party member gains 4333 XP (plus another 200 to Spark).

(The Bag of Beans description says the sarcophagus contains "treasure of the DM's choice".  I'll roll on the table counting the 4 significant encounters from the beans as threats.)

18 5 15 3

Rolling two more times for the 20: 3 19