Chastising himself for forgetting why he tries not to use his rapier, he picks up his crossbow and takes a quick look around before helping Spark get Rizo onto the chuul.

Perception: 16

Medicine (if that helps):  18

He tries to recall whether he saw the soul of either the necromancer or Rizo depart their bodies.

At the moment of striking him down, you specifically have not noticed a spirit leave the body and Rizo is unconscious but stable. Only the mummy curse keeps him from waking up. You have no problem to get Rizo mounted on the chuul. 

a few rolls:

After you finish packing up Rizo, you return towards the gateway. To your surprise, a ghostly figure in shape of the necromancer stands in the cavern exit and appears to be looking at its own hands, arms, legs and body. The figure seems to be at least as surprised as you.

(roll initiative again, please.)

spirit: 14

Spark's initiative: 19

Minty's initiative:  12

chuul: 17

(you can go first)

Initiative: Minty & Spark & Chuul || Necromancer's spirit

"Don't try to stop us.  Or we can kill you, again."  Spark makes more Magic Stones and prepares to sling one at the spirit if it takes any hostile action towards the party.

The chuul takes Rizo as far away as possible, in case the spirit is still able to cast any dangerous spells.

Minty follows Spark's example and readies an attack with his crossbow in case the spirit tries anything hostile.

"Har Har Har," it echoes the laughter of the necromancer in the cavern. "I am immortal! I knew the gateway holds great power! Finally, it is mine!" The necromancer's voice sounds disembodied but it is clearly audible. Minty is somewhat surprised to see a spirit exist in such a way. Normally they are just flitting away on the edge of his vision, but this one stands in plain view and acts quite lively and stable. Minty remembers that the Dragon Turtle outside was similar, though its shape wasn't as sharp and it didn't seem able to speak clearly, and no one else was able to see it. Clearly, Spark is able to see and hear this spirit. Might be because the Dragon Turtle was very large? Or maybe it was too far away from that gateway. It all points to the conclusion that the spirit gateway must be responsible for how the spirits act in this area. Normally, you are sure that spirits would not be able to be harmed by mundane weapons, you wonder if this is true here, too. The necromancer's spirit interrupts your thought train, "I have half a mind of letting you go since you helped me to evolve into my new form! But then I couldn't try my powers!" He unleashes a ray of greenish energy targeting Spark accompanied by sick laughter.

Attack: 19 Damage: 15 poison damage, DC 15 Con save or poisoned and if successful only half damage. 

Initiative: Minty's & Spark's readied action; then normal Minty & Spark & Chuul || Necromancer's spirit

(I rolled an attack roll, but now I realise the spell only works with a con save and doesn't require a hit. Please, disregard the attack roll and just roll the con save normally. I should mention that the spell was cast as 2nd level.)

If I'm reading it correctly, Ray of Sickness requires an attack roll and then requires a Con save if it hits.

In either case, Spark releases her Magic Stone at the spirit.  Attack: 25 Damage: 12

Quote from LaOtraVacaMorada >>

If I'm reading it correctly, Ray of Sickness requires an attack roll and then requires a Con save if it hits.

 You’re right. That was what I thought first, too. That means:

The ray clearly didn’t hit, much to the annoyance of the necromancer. Spark’s stone hits him square but it flies right through him, although it does slow down while within his matter and elicits a pained gasp. He didn’t expect to feel pain in his afterlife and Minty is also surprised at the effect. Although as expected Minty’s crossbow bolt did not find purchase despite striking true. At least some things still are normal.

Initiative: Minty & Spark & Chuul|| Necromancer's spirit

Map:

Still under the effects of Expeditious Retreat, Spark uses her bonus action to Dash to S35, going around the spirit to avoid opportunity attacks.  She uses her sling to launch her remaining Magic Stones at him.

Attack: 14 Damage: 8

Attack: 25 Damage: 11

Additionally, if at least one of these hits, she will use the power of her Rite Focus to reduce the spirit's speed by 10 feet.  She hopes that this is enough to hold the spirit's attention while the chuul runs to Q26, taking the Dodge action just in case the necromancer targets it and/or Rizo.

Minty shouts back to the Necromancer Spirit.  "The beings that have been haunting me appear to want only one thing and that is for me to gather souls and help them to cross over to wherever it is they are supposed to go.  I have refused to do so, but in your case, I am happy to make an exception."  With that he reaches into his quiver and removes one of the magic bolts stored there.  He loads it into his crossbow, takes careful aim, and fires.

Attack: 22 Damage: 14

Elven accuracy attack roll: 15

Sneak attack damage: 15

All projectiles connects with their target and clearly inflict damage in form of holes from which emanates a translucent grey smoke. Oddly, Minty notices that there appears to be a thin greyish line extending from the spirit to the gateway and then another whiteish line from the gateway to some place outside the cavern, quite possibly to the body but Minty can’t see exactly where it goes from where he stands. The line is invisible to Spark. There appears to be some connection between the body and the spirit with the spirit gateway intermittent. It also appears that the necromancer hadn’t noticed this. Anyway, the spirit does look like it took a beating, perhaps the easiest way is just to dispose of it? 

“Argh!” The necromancer yells in despair, not so much in pain. “This is not happening! This can’t be! I am invulnerable! Just leave me ALONE!” He fires another Ray of Sickness, but this time at Minty as he couldn’t hit Spark.

Attack (2nd level): 22 Damage: 17 poison damage and DC 15 save for not being poisoned on a hit

Initiative: Minty & Spark & Chuul|| Necromancer's spirit

Spark runs farther around the spirit to V36.  The Magic Stones seem to be working, so she makes three more and launches two at the spirit.

Attack: 26 Damage: 16

Attack: 14 Damage: 16

The chuul runs to O29 and does its best to smash the gateway.

Attack: 22 Damage: 13

Attack: 26 Damage: 24

"I *ill *o NOWHERE!" responds the spirit, though, at some parts its voice gave in. It looks like it has been weakened significantly with greyish fog leaking from several holes in the apparition. "Thi* *s what I w** mea** to become. You *ANNOT stop *e. AR**!" The spirit nervously eyes Spark's movement which leaves them in between its two foes. It, too, follows curiously the chuul's action unsure what to make of it.

Initiative: Minty & Spark & Chuul|| Necromancer's spirit

(Perception: 12) 

The sickly ray wizzes dangerously close to Minty.  He moves to try and intercept the line connecting the necromancer to this world.  He knows that sometimes he can focus the suffering of the dead to harm an enemy.  He thinks if he gets close enough he may be draw some of the energy away from the connection between the necromancer and whatever it is keeping him here.

Minty approaches the necromancer’s soul line which, when he tries to block it, bends around him and snakes to one side or another maintaining its connection between the spirit and the gateway. Minty tries to touch the line and it doesn’t resist or avoid contact, in fact it doesn’t seem alive. Raking his fingers through the light, Minty’s hand finds resistance as if sifting through a slowly moving beam of cold sand grains. The grains appear to be moving towards the spirit, so Minty reasons that the gateway must provide the energy to manifest the spirit in the physical realm, otherwise the spirit would have long passed on. Clearly, if one would destroy the gateway, the spirit couldn’t manifest. But then, the spirit also has sustained heavy, visible damage, and it looks like the manifestation is soon to break apart. 

The necromancer is intrigued by Minty’s curiosity about the soul link. He didn’t notice it before but is, apparently, able to see it as well, just like Minty. Prompted by its foes interest in the relic, the spirit approaches the gateway trying to avoid stepping too close to Minty. The spirit moves lethargically due to Spark’s hits and does not arrive at the gateway without a dash, but chooses to move normal and remains at 10 feet distance to the gate where it releases its ghostly hand to attack the chuul.

As it moves closer to the gateway, the soul link intensifies and thickens and moves over, not through, Minty to shorten its length. The spirit necromancer got more clearly outlined, with his features almost lifelike, when he stepped closer to the gateway but his essence remained ethereal with a grayish glow. He looks so real now after he approached the gateway, you wonder if he returns to life if he steps any closer. However much the spirit regained the features of the necromancer during his life, the damage that was inflicted on the spirit did not diminish. It does not appear able to recover its spirit force by proximity to the gateway, which you think is good news.

chill touch: Attack: 15 Damage: 18 necrotic damage

(I’m assuming that Minty’s action was to investigate the interaction between spirit and gateway.)

Initiative: Minty & Spark & Chuul|| Necromancer's spirit

Spark slings a Magic Stone at the spirit, then makes three more and launches one of them.

Attack: 18 Damage: 8

Attack: 27 Damage: 12

If at least one hits, Spark will reduce the spirit's movement speed by 10 feet, as before.

Seeing the changes in the spirit, the chuul turns and tries to attack it.

Attack: 12 Damage: 8

Attack: 11 Damage: 10

If it hits and the spirit is solid enough to grapple, then the chuul will use its tentacles (DC 13 Con save or it becomes poisoned and paralyzed).