The chuul is swarmed by insects, taking the full damage from the spell (now 60 HP).  Rizo avoids the worst of them.

It is now Minty's turn.

As the thing emerges comes into view Minty takes careful aim and fires.

Attack: 26 Damage: 23

Elven accuracy die: 14

((If weapon is still enchanted, this is without magic bolt. If weapon is not, then Minty will use a magic bolt))

Minty's bolt hits the mummy lord, wounding it (currently 49 HP).

Mummy lord's concentration: 19

The mummy lord retaliates against Minty with a Dreadful Glare, forcing him to make a DC 16 Wisdom save.

The mummy lord maintains concentration on its spell.  It uses its lair action to learn the locations of living creatures.

Spark moves into view of the mummy lord, staying behind the insects.  She launches two Magic Stones, although it's hard to hit with her sling at this distance.

Attack: 11 Damage: 9

Attack: 15 Damage: 11

Both of Spark's Magic Stones miss.

The chuul Dashes twenty feet back, just out of the mummy lord's Insect Plague.

It is Rizo's turn.

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

Rizo and Moni move into close combat with the mummy lord where Rizo unleashes a Green-Flame Blade. Then, he instructs Moni to fiery teleport them 15 feet back towards the chuul and they both move the rest of the way out of the area of insect plague.

Attack: 23 Damage: 5 magical bludgeoning plus 16 doubled fire damage

Moni’s teleport: 10 doubled fire damage unless DC 15 DEX save

I’m not sure about the distances. With this move, Rizo could move a total of 45 feet (30 normal movement and 15 teleportation). 

If I'm understanding the situation correctly:

Rizo was waiting for the mummy lord outside the pyramid, which is a 60-foot square.  The mummy lord was in a sarcophagus in the center of the pyramid.

The mummy lord emerged from the sarcophagus and cast Insect Plague, centered on Rizo, while remaining at its current location.  Rizo's 30 feet of movement is just enough to get to the edge of the Insect Plague, unless he Dashes or can ignore difficult terrain somehow.  At that point, he would take damage again but Moni could teleport him somewhere else from outside the Plague.

I don't see any way for him to reach melee range this turn, though.

Quote from LaOtraVacaMorada >>

The mummy lord dodges Rizo's fire bolt and moves 20 feet towards him before casting Insect Plague on Rizo. 

Quote from LaOtraVacaMorada >>

If I'm understanding the situation correctly:

Rizo was waiting for the mummy lord outside the pyramid, which is a 60-foot square.  The mummy lord was in a sarcophagus in the center of the pyramid.

The mummy lord emerged from the sarcophagus and cast Insect Plague, centered on Rizo, while remaining at its current location. 

 I thought it was just 10 feet away.

Okay, I stand corrected.  Rizo can reach the mummy lord.  We can assume that the mummy lord cast Insect Plague to exclude itself from the area, so Rizo could also advance far enough to get out of the Plague, if he passes the mummy lord's position.

However, the teleport, which is only 15 feet, is not enough to get past the plague if he is trying to move outside the pyramid.  He could move either to the side (within the pyramid), or farther inside the pyramid.

You’re right, Rizo can’t move after Moni’s teleport because t he still teleports as much to the exit as possible even if he remains in the insect plague, my bad - poor choice of an action. That means he needs another Con save, right? 
Still Koni should be able to fly out - 10 feet to the mummy and away from the center, then 15 feet back by teleport which should place them at 15 feet away from the exit side of the plague area limits but she has 20 feet left. 

Rizo con save: 7

The mummy needs a Dex save, too.

Mummy Lord's Dex save: 10

Insect Plague damage: 29

Mummy Lord's concentration: 20

The mummy lord is injured by Moni (39 HP), but it maintains its concentration.  It uses another legendary action to target Rizo with a Dreadful Glare, forcing him to make a DC 16 Wisdom saving throw.

Rizo Wis save: 26

I should also do two concentration checks for Minty's Magic Weapon spell, one for the previous plague damage and one for now:

DC 10 Concentration: 7

DC 15 Concentration: 10

Minty Wisdom Save: 6 

I think it’s the mummies turn still

That was Minty's save from the last turn.

Quote from UncleDobbin >>

That was Minty's save from the last turn.

 

 Oh, I see. This mummy is considerably harder than the last. One poor decision on Rizo’s part and he’s got chances to go down right away.