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: 28 Damage: 23

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 31

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 24

Arcana  7

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

(lol, I was waiting for a Dune reference.)

Tempe looks upon the destruction, follows the worms path, not from where it came from, but where it was going too. He looks at the building it was about to destroy, wondering if this is some kind of power struggle. (Maybe, it's the spice 💩. )

Tempe suggests to Randle, while he follows the trail, "Maybe, you should gut the poor creature to find out where it's been, by, what it's ate. I think it's poison may be valuable, too."

Randall:

”Of course, good idea,” says Randall.  He guts the worm first to let out anything inside its stomach, then starts working carefully on the stinger tail.

Survival:  21

"Thanks to your healing I am" Pepin answers Randall After which he helps Randall with his tasks of both healing and taking care of the worm.

(I'm not sure if you're only supposed to get one dose or more but since it's near the end of the game I'm going to say you harvest 3 doses of purple worm poison.)