The sarcophagus contains a Necklace of Fireballs with one extra bead, another spell scroll, and 358 gold pieces.

"Well, we have four beans left.  Want to finish planting them?"

The Necklace of Fireballs has a total of 7 beads.

Rizo’s Con save: 12

(I believe the mummy was worth 15000 XP because it was in its lair, right? It says so just below its stat block.)

“”Sure, let’s do them all.”

While waiting for the last few beans to sprout, Rizo eats a handful of 20 Goodberrys (he has big hands). 

(Sorry for the delay; I've been busy with real life things.)

Bean roll: 62

A nest containing 6 eggs appears.  These eggs have the potential to permanently increase someone's lowest ability score, but are also quite dangerous to eat.

"I think if anyone wants to try eating these, we should wait until we're safely back in town."

Bean roll: 18

Another pyramid appears, right next to the last one.

"What do you say?  Same strategy as last time?"

(After rereading the Mummy Lord's text, I agree that we should each have had 5000 XP instead of 4333 for the last one.)

"I'm not sure. I had the feeling that hiding didn't really work. That thing knew exactly where I was. Perhaps we can just march in and take it from there?"

"I thought we lured it out last time because someone said that they are stronger in their lair."

“Right. Well, then I can wait outside near the chuul and don’t need to hide under water like a scared carp.” Rizo and Moni move next to the chuul, then Moni enters to anger the mummy and to draw it out. At first opportunity she shoots at it and hurries back to the chuul. Meanwhile, Rizo readies a Fire Bolt to be released at the sight of the mummy.

Attack: 12 Damage: 18 doubled fire damage.

As before, the mummy lord notices the destruction and emerges from its sarcophagus.

Mummy lord's starting HP: 99

Mummy lord's initiative: 9

Spark's initiative: 8

Chuul's initiative: 2

Rizo’s initiative 9

Minty's initiative: 19

Just now I realized that Moni might have had advantage on her attack as she had waited for the mummy. If this is the case the attack roll would be: 17

I'll say that Moni had advantage, so the Fire Bolt (just barely) hits.  

Initiative order:

20: Lair actions
16: Spark
15: Chuul
13: Rizo
9: Mummy Lord
6: Minty

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

Spark stands about 30 feet from the entrance, to the side, and readies a Magic Stone to fire at the mummy lord if it enters her view.

The chuul stands to the side of the entrance and prepares to attack.

The mummy lord emerges from the sarcophagus.  Rizo may use his readied action.

Rizo releases the spell at the emerging Mummy Lord.

Attack: 14 Damage: 12

The mummy lord dodges Rizo's fire bolt and moves 20 feet towards him before casting Insect Plague on Rizo.  Both Rizo and the chuul must make a DC 17 Constitution save, taking 17 piercing damage on a failed save, or half on a successful save.

Chuul's saving throw: 5

Rizo Con save: 18

Whose turn is it now?