Spark startled when the goblin stepped into sight and immediately realised that it was an illusion, she launched her magic projectile far off. But before the goblin could turn around to alarm his companion, she was dead on target for her next two attacks dropping the first goblin to the floor. 

The chuul (you shouldn't look here just yet)

Map (Minty, Bappy and Rizo can still act in the surprise round):

Minty takes careful aim at goblin #2 (the one with the rod).  He fires his crossbow

Attack: 20 Damage: 8
Extra attack die from Elven Accuracy: 9

Sneak attack damage:  9

Minty Initiative: 20

Bappy's Initiative 9

Bappy tries to sneak around the rock to J25
19

Minty headshots the goblin, he staggers a step back and then topples over, the rod rolling out of his hand onto the floor. The last goblin turns around just in time to see Rizo stepping around the boulder with Lily on his shoulder. Both shoot a fireball at the remaining goblin. Bappy moves silently the other way around the boulder.

Attack: 8 Damage: 12

Attack: 25 Damage: 6

Rizo initiative: 15

Goblin: 10

Quote from cyreon >>

Attack: 14 Damage: 3

Attack: 19 Damage: 4

Rizo initiative: 5

Goblin: 14

Keeping a reference for the rolls, because I need to edit the original post.

The goblin turns to find two fire bolts coming his way, he can deflect the first but gets slightly burned by the second. The fire itself does near nothing to him but the full view of the scene turns his stomach. A gnome standing over his dead mate and his leader, with a bolt protruding from his forehead, undoubtedly and expertly taken out by the elf that now trains the crossbow at him. In a snap decision he jumps into a run as if his life depends on it. He rounds the boulder towards the crevice disturbing the chuul, but quite likely he sees the chuul as the smaller risk or even a tactical advantage. 

(The goblin takes a dash action and moves to T29.)

Minty's turn (but I think this round you can all take your turns in any order, it will not matter, next turn the chuul will join at its initiative)

Initiative: Goblin #1 (14 - fleeing), Minty (12), Bappy (9), Rizo (5), Spark (4), Chuul (11)

Map:

Minty moves to F5 and holds an action to shoot at anything that should appear and threaten Spark.

Under cover of the wall's shadow and therefore in dim light, Minty is confident that he can hide from the chuul even in plain sight. When the chuul comes into view he notices that it is posed to follow the goblin that had just dashed in front of it. Though, Minty can't get a visual of the goblin himself. Bappy, hidden from the goblin, sees it running along the extend of the wall, probably trying to save its hide. Will you try to take it down or let it go? Meanwhile, Rizo approaches Spark and the fallen rod that the goblins had prepared, ostensibly for use against the chuul. Rizo, proficient in theory about the arcane arts, offers his help to Spark to investigate the rod. Perhaps, together they can learn how it works or what exactly it does.

(Spark may, if she wishes to do so, either investigate the rod to figure out how it might work (Investigation), or try to study its arcane power - with Rizo's help - and conclude how to use it (Arcana with advantage). Both are tiered checks where you obtain more information the higher the roll is but you have to choose one of the two for this turn. Bappy, what's your plan? Attack the goblin, or let it go? He hasn't seen you and wouldn't know whether or not you are associated to the attacks on his companions, too, but he is scared.)

Bappy's turn but Spark may act already, too.

Initiative: Goblin #1 (14 - fleeing), Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

Map:

Spark returns her sling to her belt and picks up the rod.  She tries to investigate it to see if she can figure out how to use it.

Quote from LaOtraVacaMorada >>

Spark returns her sling to her belt and picks up the rod.  She tries to investigate it to see if she can figure out how to use it.

Please, roll Investigation normal, or Arcana with advantage (sorry, I was editing my previous post while you posted and got busy so it took me long).

Arcana: 9

The rod, that the goblins had used for their ritual, is intricately carved from a bone that could have belonged to a large fish-like creature. It appears to be a relic now infused with the goblins’ magic. You can't ascertain what exactly it is or what the goblins wanted to use it for against the chuul. You do believe that you can access its power and try it on the chuul but you also think that you may not be able to tap its full potential without understanding it better first. 

(You may use the rod as it is next turn, or try one of Investigation or History or Religion to learn more. Someone with proficiency in a skill may help you to give advantage. Bappy's half proficiency would be efficient half of the time, so if you use that feature roll an additional d20 and only get advantage on a 11+.)

Bappy's turn, then we start the next round.

Initiative: Goblin #1 (14 - fleeing), Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

Bappy stays still and watches the goblin run.

With a quick glance the goblin looks back if anyone follows him but thinks it best to continue to run and meld with the shadows.

Minty's turn

Initiative: Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

The chuul initially watching the direction in which the goblin had run, now turned sharply towards Spark. Even though, it cannot possibly have a visual with the large boulder blocking the view. All of a sudden it turned around backwards and, with two powerful thrusts shot, or actually swam, over the boulder towards Spark and by happenstance, Rizo. There must be something on Spark that drew its attention, its tentacles writhing from its mouth almost as if it found something delicious. 

(I'll take it that Minty stayed put. Now Minty gets an attack of opportunity, when you can. Meanwhile it's Bappy's turn.)

Initiative: Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

Map - round 3:

(I will typically wait one day for a response and then continue with the story or combat. Should you need more time, I would like you to let us know.)

Rizo slams his club at the charging chuul and gives signal to Lily to teleport them away (F11), letting Spark join if she wishes. 

Attack: 12 Damage: 12

DC 15 Dex save or 4 fire damage from Fiery Teleportation, Dex Save: 1

Spark's turn (But until it is their next turn, Minty and Bappy can still post what they wanted to do during their turn.)

Initiative: Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

Spark teleports with Rizo and then runs to D6, trying to investigate the rod as she goes.

Putting this in spoiler tags in case someone is considering helping her:

Rather than a magic item, the rod appears to be a conduit, or catalyst, that empowers or influences magic cast on it. You know that the goblins were about to use it at the chuul and chuuls are minions of Aboleths. It is very likely that you hold an Aboleth’s bone and that the goblins use its essence to control chuuls. You feel that you are at a breakthrough point, with something else still escaping your full understanding of the rod. At this stage you feel comfortable that you can use the rod to control the chuul as the goblins had planned, or nearly as good. You can try to learn more but you would need more time.

(You are confident that you can control the chuul with this rod until your next long rest, you could improve on that if you spend another turn studying it with one of the other options. Having learned much already you will get advantage on subsequent rolls but you need to roll higher to learn something new. To use the rod to control the chuul is an action.)

Minty‘s turn (Bappy may still post his turn until he’s up again)

Initiative: Minty (12), Chuul (11), Bappy (9), Rizo (5), Spark (4)

Map round 4:

((Sorry, life got a little nuts))

Minty is equal parts calmed and dismayed seeing the goblin souls depart as they died.  He sees his companions fiddling with the rod and is curious as to what they have in mind.  Not wanting to risk any of the others, he takes careful aim and fires.  

Attack: 17 Damage: 16

Elven accuracy die: Attack: 24 Damage: 0