Guile sends his flaming sphere down into the tunnel after his companions hop up, blocking the intruders.  DC 16 Dex or 7 fire

He readies a Fireball when another Umber Hulk emerges, guarding his companion's retreat.  Guile smiles cooly,  "Hellfire is with us."

29 fire DC 16 Dex half

Aegnor will fire an eldritch beam at any umber hulks make it through the fireball.

MOVE: Aegnor will attempt to move up to 30ft, just so he can see any Umber hulks that get through the fireball.  Preferably away from umber hulks but still in range. (spending 30 feet of movement; first diagonal costs 5 ft. of movement, second costs 10 ft., third costs 5 ft., etc.)
BONUS: Command Skelly and Shadow to move with him or as best speed for Skelly
ACTION: Cast Eldritch Blast,  Eldritch #1 Attack Umber Hulk#1: 15! Damage: 6 Eldritch #2 Attack Same Target or Next umber hulk: 16! Damage: 11

End of turn!

Reaction:  Opportunity attack if it presents itself or spells

Between Aegnor and Guile the next Umber Hulk goes down!

Arnel returns with a 1/2 dozen guards.  He orders them to get a rope to down to the others below.

Everyone roll a hearing perception check.

Guile perception: 14

Hearing 21

Aegnor Perception:  19

Vanja perception: 28

Aegnor & Vanja hear a rumbling coming closer from DEEP down in the earth.  Something BIG is coming up.  A frenzied chittering from the remaining Umber Hulks seems to convey a sense of terror from the creatures!

Arnel also heard the ensuing chaos and declares, while making washing motions with his hands ending with his palms out front,  "Nope! I'm out of here!"  He turns around and exits the building.

What is your decision for the party Guile?

Guile shouts, "Move out of the structure and fall back!  Prepare for ranged assault!"

Guile hurries outside and points to the guards to make a wide perimeter around the structure.  He wills his flaming sphere to follow 30' behind.  "Ready archers!"

"There's something very big down there!" Vanja calls. She falls back with the rest. She stows her shield and rapier and prepares her longbow and arrows. 

Aegnor falls back with Skelly and shadow heeling, "Come."

Backing out Aegnor will use Eldritch blast on any monsters coming out of there.

MOVE: Aegnor will attempt to move back 30ft to exit the building: (spending 30 feet of movement; first diagonal costs 5 ft. of movement, second costs 10 ft., third costs 5 ft., etc.)
BONUS: Command Skelly and Shadow to move with him or as best speed for Skelly
ACTION: Cast Eldritch Blast,  Eldritch #1 Attack monster#1: 24! Damage: 15 Eldritch #2 Attack Same Target: 13! Damage: 10

End of turn!

Reaction:  Opportunity attack if it presents itself or spells

Ravager will retreat with the others, though (since she didn't hear anything) she'll look puzzled until Vanja mentions "...something big..."

The following worm like creature burst from the ground:  Initiatve 13

New  round, roll for initiative!

Arnel : 8

Guards: 20

Ravager 17  Initiative

"What in the Nine Hells..."  Guile init 9

Nature 9 (assuming it's a Monstrosity)

Vanja wishes she hadn't put her trusty shield aside when she sees the wormlike creature waving its teeth at them, but prepares to loose some arrows. Initiative 21

Aegnor "That's a beastie for sure...Let's kill it."

Aegnor Initiative:  18

Initiative 

Guard                           20

Vanja                            14

Ravager                        13

Worm                            13

Guile                                9

Arnel                               8

Aegnor                            7