Muck's goo starts to shimmer and sparkle as it looks at Errk and the other goblins, saying, "See? Isn't it nice that we're friends now and we can all help each other? We're going to help you take the head off of Klarg guy's body. You're going to help Sildar be safe...

...But, oooh! Wouldn't it be even nicer if you let us bring Sildar with us right now! Then he can help us take off your boss's head and it'll be that much more fun for everyone! It'll be so exciting! We'll come back and laugh about how silly it was that we were trying to make each other dead earlier! Ha ha ha..."

Shimmering Muck is using its Fey Presence ability on the goblins, requiring a DC 13 Wisdom saving throw against being charmed.

As Muck begins to activate their Fey power, Penley, drawn by the sound of battle and negotiation, creeps along the tunnel, approaching from the side tunnel. He sneaks up, bow of radiance in hand. He sneaks up, and starts to observe.

His instincts are telling him to attack, to wipe out the filthy defilers. And goodness, are they filthy. Ugh, that smells horrible. But! To rush in now would be to doom that human. And if that happens, Penley isn't getting paid or becoming famous. Which will not happen. 

Instead, he waits in the shadow, clinging to the wall, ready to strike. He takes the Ready, with the trigger of: 

one of the goblins performs a hostile action or drops Sildar

If the trigger activates, he will cast Eldritch Blast on the offending goblin. If more than one goblin is the offender, he shoots the one holding Sildar.

Eldritch Blast Attack VS. Goblin: 11 to hit, 1 Radiant damage

OOC: You might ask why I would potentially shoot the goblin holding Sildar, when he would fall, but just wait.

As Penley hides in the shadows, he whispers something to his weapon. Now, his bow is unique in the fact that it possesses three strings instead of one. He activates the celestial power of his weapon, causing one of the faint, golden strings to detach from the weapon, and snaking forward stealthily so as not to be seen.

It slithers through the eating cave, and subtly absorbs into Sildar, as Penley activates his Healing Light ability using his Bonus Action, healing Sildar for 4 HP of healing.

Quote from Mister_Whisker >>

Quick question: how is a goblin holding an adult human over the edge? That seems... a  bit challenging from a physics standpoint.

 No idea. The module says they do though (and provides that picture at the same time, probably depicting him after/if you save him/get him released). I am going to say that the human is slightly on the ledge so not all weight held by the goblin, but enough leant forward that if they let go he will fall. Plus, he has been beaten to within an inch of his life, and probably starved for a while too (though only a day or so max based on timings) so isn't massively chunky. 

I will assume that Muck moves to a point that he can catch all 4 surviving goblins in a 10ft cube (there is one. Standing on the corpse of one of the other goblins) They have arrows pointed at him when he moves in case he does something threatening, but he doesn't. Surprisingly, all 4 fail their saves. You don't get to choose the creatures though, so technically Bridgette and Bok'shir need to make saves (though charming them won't affect anything mechanically unless they wanted to attack you). 

This doesn't give you automatic success, but does give you advantage (as per the charmed description) which I rolled on your sheet, and got 2 8s (so 11 total). For full transparency on this check, I will roll here. 0

Penley, that healing seems to give him a lot more strength than he used to have, enough that he has a decent chance of surviving that fall which could otherwise have been deadly (though he is important enough for death saves)

The goblin thinks about it for a second. "He is too weak to fight Klarg. We will keep him safe here. And you go deal with Klarg." Sildar groans and lifts his head. "His word means nothing, don't trust him." "Silence human!" and the goblin goes to slap him, but then stops, realising that would go against the deal of keeping him safe. And instead pulls him back from the ledge. (Penley would an almost slap count enough as a threatening action to trigger your action?)

(Is Sildar no longer hanging over the edge then? Could Muck position a sleep spell to catch the one holding him without him falling and without catching allies?)

OOC: the almost slap would not count. I meant only if the goblin was to do something that would probably cause actually HP damage, such as gut Sildar or drop him over the edge.

When the goblin tells Sildar to shut up as he tried to warn them not to trust it, Bok'Shir's eyes narrowed. It was unusual for humans to be treated so by a goblin, but even so he recognises someone who is helpless. Down trodden.

He grasps his sword tighter, and starts to gather the flames ready to strike out at the one next to him.

"Not trust Yeemik. If Yeemik want Klarg dead, Yeemik get own hands dirty. My hands not get dirty for Yeemik", he says.

Quote from MAOShea >>

(Is Sildar no longer hanging over the edge then? Could Muck position a sleep spell to catch the one holding him without him falling and without catching allies?)

 Correct. You can now. It will also affect Sildar. Who is definitely going to sleep as it affects lowest hp first. And the ones with the bows also look weaker than the one holding him. Though saying that. It says a point within range not a point you can see so you can position it within the rock and only catch the one goblin if you wished to maximise the chance of them being the one to sleep. 

I will wait to hear about the outcome of a potential sleep spell before resolving all of this.

Without a visual, I'll do my best. Sorry if I'm off here, but Muck will definitely cast the spell, trying to angle it so it gets the gob holding Sildar and as many of the others as possible without catching Sildar himself. If the others fall asleep and the one goblin is still up, thats a situation Muck could live with.

Cast Sleep for a total of 23 HP worth of snoozies.

You can't catch the goblin on the floor without catching Sildar. But the three up the top you can. The two bow creatures fall asleep. The other does not.

You are back in initiative. Bok'shir take your readied attack.

The one with Sildar having seen you drop the two with bows will go to push Sildar off the edge. Penley attacks and misses, and Sildar falls. He does survive the fall though thanks to the healing recived.

The one next to Bok'shir will be a bit surprised so doesn't take an attack this round.

Everyone can take an action (Everything before this message, including Bok'shirs attack was last round/reactions) 

Seeing everything degrade into violence once again, a wicked grin spreads across Bok'Shir's draconic maw, as he swings his flaming blade at the goblin next to him.

And he doesn't stop there, he reverses the blade for another flaming swipe, but not before his eyes grow black and a dark energy spreads from them into the goblin.

Reaction: Attack goblin next to him with Green-Flame Blade - To Hit: 4+5=9 (assumed miss)
Bonus Action: Hexblade's Curse on goblin next to him - +2 damage, crit on 19-20, regain 4 HPs if target dies
Action: Attack goblin next to him with Green-Flame Blade (with Hexblade's Curse)- To Hit: 1+5=6 (assumed miss)
Special: Hexblade's Curse - 1/10

Bridgette sighs.

"I kind of hoped we wouldn't need to keep trying to murder each other... but I guess that's where this is going," she mutters.

She targets the goblin who just dropped their contact with a beam of eldritch power, using a bonus action to move her hex to him as well.

Eldritch Blast
Attack: 19; Damage: 10 (force) + 3 (necrotic, from hex)

If the goblin is still alive after the attack, it must make a DC 13 Wis save or be frightened of Bridgette until the end of her next turn.

OOC: let’s go! I knew the healing was worth it!

Penley leaps out of the shadows dramatically. He rushes forward, calling out to Muck “Get the human out of here!”

He draws back his bow, and fire the arrow at any of the remaining goblins, preferably Yeemik, if the little punk is still alive.

Eldritch Blast Attack VS. Goblin: 14 to hit. Hit: 10 Radiant damage

(Damn, the guy holding Sildar was actually my priority, but all good. I'll chalk it up to Muck not being the best at this and this is a fun way for things go to anyhow!)

Following Penly's command, Muck will race to Sildar's side (Dashing if it must) and start the process of pulling him to a safe corner to wait out the battle. In the process, it will try to fire off a blast of arcane energy towards the one awake goblin.

If Muck can make it to Sildar without a Dash...

Eldritch Blast

To Hit: 21

Damage: 9

Yeemik is still alive. But injured. And currently frightened of Bridgette. Muck to go

(Posted above. Maybe it was missed.)

(It was a case of me loading the page up before you posted. And it doesn't automatically update.... And I know they were your priority, but the spell targets from lowest health first, and they have more health than the bow ones)

There is also two awake goblins, Yeemik and Errk. I assume your target is Yeemik for dropping Sildar. Who now drops off the edge himself, dead. 

Errk will run over to Muck to try and get revenge. 21 to hit, 3 damage. Followed by a 6 to hit. 

Everyone is up

OOC: Rinse and repeat! 

‘The cat-man chuckles at the foolishness of the creature, then attempts to skewer it with an arrow of divine fury

Eldritch Blast Attack VS Errk: 13 to hit. Hit: 9 Radiant damage

(OOC: If Errk moved to get to Muck, does Bok’Shir get an AoO?)

(Also, is Muck in melee with Errk now? That will determine it's action. Sorry for all the miscommunication, but without a map I'm kinda running blind here.)