Randall:

Even with his new shield, Randall’s defenses are as nothing compared to the purple worm’s power!

Randall Dexterity Save:  16

Pepin would us his reaction for flash of genius to shout out a tip for Randall to evade the Purple worm (+5 to the save)

Randall:

Randall’s smile of thanks to Pepin is interrupted by a high-speed stinger!

Constitution Save:  4

(Hopefully the Purple Worm takes damage as it plunges through the wall of fire to attack.  How bad is it looking?  Randall himself is bloodied already!)

((It should probably take damage at the end of its turn: "One side of the wall, selected by you when you cast this spell, deals 5d8 Fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there."

Without a save it seems))

Zenodor moves closer and tries to magically rot and shrivel the worm. Closing to 30ft and casts Blight on the worm

46 necrotic damage, DC 21 CON for half

Tempe looks on with concern, but still concentrates on the fire wall, On his turn, he points his new toy at the creature, if he can he'll aim at the mouth hoping for a reaction. As long as he doesn't hit his friends, or innocents.

Fire wall, deals 5d8 =25 fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.

Lightning Strike. You can use an action to cause a bolt of lightning to leap from the staff's tip in a line that is 5 feet wide and 120 feet long. Each creature in that line must make a DC 17 Dexterity saving throw, taking 9d6 = 30 lightning damage on a failed save, or half as much damage on a successful one.

Randall:

Bleeding and pale but holding his ground, Randall causes his Spiritual Weapon to attack once more, then spreads healing on himself and the others fighting this raging behemoth.

Bonus Action Spiritual Weapon:  Attack: 20 Damage: 16

Action:  Mass Cure Wounds heals all allies (and potentially other still-living victims of the purple worm) a maximized 36 hp.   Blessed Healer gives Randall another +7 hp healed.

Purple Worm's CON save (vs. Blight): 13 

Purple Worm's DEX save (vs. Lightning Strike): 2 

Tempe can roll damage from his wall of fire spell twice, once for when the Purple Worm ploughed through it to get at Randall, and again for when it ended it's turn within 10 feet of the fiery wall.

The Purple Worm was bloodied after moving through the wall of fire to get at Randall. (It currently has less than a third of it's starting hp.)

(Pepin is up.)

Fire damage 21

I can count that damage roll for both instances ok?

If so the purple worm looks to be clinging to life and with all that's happened since the siege of the Goblin Horde you can surmise (without an investigation check) that this monster, much like the black dragon and later dracolich, were or are being controlled by some greater entity.

Pepin activates his force cannon as a BA:

To hit 22 for 9dmg
He than casts booming blade and hit with his warhammer:

To ht 11 for 15dmg

Luckily the force cannon happens to be enough to slay the purple worm. It's death is somewhat anti-climatic as it's last roar fades into a low, groaning death rattle and then it's bulky corpse is just laying there.

(This combat is over.)

Tempe dismisses his spell.

Randall:

”Is everyone okay?” Randall asks with concern.  Noting that the party members all appear to have survived, he backtracks the purple worm’s path of destruction to find any survivors who may be in need of healing.

Once the dust has settled and the wounded are seen to, Zenodor starts to examine the corpse of the beast.

"I never heard of such a monster near a city. I believe they live in deserts. This must have travelled far, and I cannot imagine it did so unprompted. Someone or something must have surely guided or ordered it to come here to attack us."

He examined the corpse, looking for signs of control like perhaps chains or manacles, or a magic item, or perhaps traces of a control spell.

Investigation 18

Arcana  20

(the desert planet of Arrakis seems a long way. I think we need to find out who sent this thing)