Tallanvor waits in a concealed location for Humbolt to charm the thugs. He positions himself to throw a net. If they notice him before Humbolt has a chance to 

Stealth: 6 + 10
Initiative: 10
Strength for net 9 +2 (rolled with + 1 rather than a +3 for attack)

After helping to drag the prisoners into the northern cells Ro ducks behind a sarcophagus and prepares for the ambush.

Stealth 19

Initiative 18

As soon as Humblodt casts his spell she'll dash forward and try to wrap herself around one of the targets' faces so Twiglet can get in with the manacles.

Grapple 19

Edit: Finding herself unexpectedly being peeled from the bugbear's face, Ro pulls out a second sword and starts to let the rage out.

Twiglet quickly strings a tripwire across the entrance - Thieves' Tools: 17

Then he finds a hiding spot and and pulls out the manacles

Stealth: 18

Initiative: 12

As soon as Ro latches herself to someone's face he'll run out and try to trap them with the manacles.

Acrobatics: 25

Then he'll use his bonus action to try to put manacles on Tally's dance partner.

Acrobatics: 22

That done, he'll run back to the sarcophagus and duck behind it.

Around twenty minutes after they left, you heard the voices of Horace and Jasper returning, laughing and joking. With them are two more Redbrands, but more importantly towering over them is a bugbear smiling away. You recognise from it's armour that it is from the same clan as those in the Wave Echo Cave.

One of the new Redbrands trips over the tripwire 5 but regains his balance. 

As they move closer, Humblodt casts Charm Person on Redbrand 1 2 who is unaffected. Tallanvor's attempt to throw a net over one of the new foes fails too.

Roars jumps on the Bugbear and tries to grapple it's face. The Bugbear trys to pull her off 6 succeeding.

Enemy Initiative 5

Twiglet aims at the Bugbear as it pulls at Ro

Attack 20 (15+5)

Damage 15 (from discord with plus 3)

The bolt flies true hitting the bugbear in the chest. It roars in pain, tearing the arrow out. "This was a trap! You will pay traitors!"

It swings it's club at the surprised Horace Attack: 20 Damage: 17

But the attack misses. 

The two Redbrands have decided similarly about Jasper

Attack: 19 Damage: 4

Attack: 10 Damage: 5

Attack: 6 Damage: 8

Attack: 14 Damage: 3

Only one manages to land a blow opening Jasper's arm. The two charmed thugs run for their lives towards the bottom of the room taking disengage. "You can't escape!" 

Tallanvor Blesses Twiglet, Roars, and Humbolt and watches the chaos unfold.

Twiglet aims again at the bugbear as the brute scans the room

bless effect 1

 Attack Attack: 18 Damage: 17

Humblodt points his staff at the melee, and the retreating Horace and Jasper, uttering some really quite vile sounding words involving Titchmarshing and Dimmocks. Grasping weeds and vines sprout from the floor and around the sarcophogeese sarcophogi, twisting the floor into a botanist's wet dream, or worst nightmare.

DC13 Strength Save for all within (assuming all bad guys plus the unfortunate Ro, who at least gets to use +1d4 from bless on her save).

As a bonus action, Humblodt will also cast Shillelagh, in the unlikely event he hadn't done so during preparation.

Ro manages to pull free of the vines. If Horace and Jasper are restrained she'll attack the bugbear. If not, she'll chase after them and attack whoever successfully saved - preferably Jasper since he's already injured.

 1st Reckless Attack: 18 + 1 Damage: 7
Offhand Reckless Attack: 15 + 4 Damage: 8
Radiant Damage (On first attack that hits): 7

Steve Strength Save 8

Walter Strength Save 9

Grommett Strength Save 15

Horace Strength Save 6

Jasper Strength Save 18

Steve the Bugbear, Horace and Walter are all trapped within the grasping vines. Roars slips between the weeds, slicing at Jasper, who collapses wounds glowing. She rounds on Horace hitting him with her other sword. "You tricked us!" He shouts as realisation kicks in.

Another arrow strikes Steve who falls silent.

Steve strength check 21 Because Steve is already down

Walter strength save 12

Horace strength check 11

The restrained enemies break free looking furious, Horace looking bloodied.

Grommet attack Roars 

Attack: 7 Damage: 6

Attack: 20 Damage: 8

 Ro shouts "SURPRISE!!!"  and slashes at Horace again.

First Reckless Attack: 23 Damage: 4

As Horace drops she turns to Grommit and smiles horribly before attacking. [I'm assuming the 1st attack + radiant damage will be enough]

Offhand Reckless Attack: 24 Damage: 3

Radiant Damage (add to 1st attack that hits):  6

"Steve", calls out Humblodt hoarsely before assailing him with his shillelagh, "I never did like you".

Attack: 18 Damage: 9

Forgot to add the 1d4 for bless: 4 => 22 to hit.

Horace also falls to the furious goblin. Grommet is sliced too but remains standing.

Seeing that Steve is unmoving, Humblodt's swaps his blow to hit Walter, who stumbles but is still standing. "I don't even know who you are" he shouts.

"I don't know who you are either", growls the dwarf, "but I do know I don't like you either!"

Twiglet takes aim again from behind the second sarcophagus. He can feel the affect of Tallanvor's prayer, he feels light and alert 

affect of Bless 3

Attack: 13 Damage: 12

and is "lucky"

Attack: 28 Damage: 15

Grommit approaches Roars, then suddenly go limp falling to the floor, an arrow sticking out of his back.

Twiglet gives out a little whoop.....and hopes no one else noticed what happened. The bolt had actually gone in completely the wrong direction when he slipped slightly on something best left unknown that was leaking from the sarcophagus. The bolt was heading for Humblodt, but struck the helmet of one of the skeleton's and ricocheted into the bugbear. He wouldn't be able to pull that trick off ever again if he tried