When Leelan (and Grugga, possibly?) slips into the cave opening, he sees the following:

(The white circle is the range of any portable light source brought with you.  The larger circle is the limit of darkvision)

The ceiling rises quickly until it beyond the reach of your darkvision.  The floor inside the cavern is mostly flat.  In some places it is rock, but is other places it is stone and loose soil.  It looks like someone has put some work in to level the floors.  There is indeed a small stables set up near the cave opening, with at least three horses inside.  They whinny nervously as they cannot see but sense your approach.  They are agitated and perhaps with good reason... in addition to the pungent odors of a stable, there an odor of decaying flesh inside the cave, much stronger than what could be detected outside.  In place of a gate, there is a ten foot length of chain connecting two of the posts which looks like it could be removed.

For Leelan (and anyone with him):

Grugga remains at the mouth of the cave. He stands off to the side and pulls Zip from its resting place, and tinkers the bottom of it to shine a 10 ft radius of light (the other hydrospirator stops giving off light). He then orders it to hover 10 ft above the very top of the cave entrance (40 ft above the ground) so the light can't be seen from inside the cave. He gives it no instructions except to wait for instructions.

Grugga also extracts a bag of Ball Bearings (bag of 1,000) from one of his pouches full of knickknacks. He prepares to spread them in the entrance in a flat, smooth area with less loose debris on the ground.

The item says it can fill a 10 ft square (4 5ft squares), could I spread it out in a 15ft long, 5ft deep line (3 5ft squares)? That way I could cover the entire entrance instead of 2/3 of it.

He keeps his axe sheathed for now.

Moving further into the entrance of the cave, Leelan goes over to the chain, as quietly as possible. Once he reaches it, he tries to remove it so that the horses can pass. He then pulls out some dried fruits from one of his rations and tries to lure the horses out of their pen.

Animal handling?:  14

Stealth (if I need to roll again):  16

Leelan creeps up to the stable gate, quiet as a mouse.  He is able to remove the chain and lay it gently on the cave floor without a sound.

Horses are complicated creatures.  It takes little convincing to get them to move and they seem familiar enough with the environs that even in the dark, they start to move through the open gate and out the mouth of the cave.  They nearly bowl Leelan over in their determination to get outside.  They are not particularly quiet about it, however, as they continue to whinny nervously and their combined footsteps (it turns out there are four horses inside) make a fair amount of noise.

As Leelan exits the cave behind the horses, he hears something moving in the dark.  There is no attack, but it sounds like something has been stirred and is now moving about.

Once outside, the horses neigh in irritation at the soaking rain but will not return into the cave.  You can either try to contain them somehow or let them wander about on their own.

(There are a couple minutes left before the elf said he would begin his attack.  Here is a map.  Please indicate where you would like to be located.  We will start the assault tomorrow.  Anyone who hasn't indicated where they want to be will be placed based on DM best guess at where you would want to be.  Grugga, yes you can set ball bearing so that the cross then entire path out of the cave.)

Rohuzar will go in with Leelan and Grugga, quietly trying to get close enough to identify the humanoid shapes more clearly. 

no lights. We’re sneakin’ lol

stealth 26 
perception 12 

Crap, I was slow to post.

Rohuzar will be right at the entrance to the cave, just beyond the ball bearings

(No problem)

Rohuzar:

Grugga will hug the cliff wall, right next to the cave entrance on the left. He will deploy the ball bearings once all our allies and the horses are outside.

Grugga won't do anything with the horses, but 4 is a pretty lucky number. Maybe someone with better animal handling will wrangle em up to use as mounts.

((Also I have to retcon tinkering Zip. It only works on magical objects - he is technically magical and a creature - so no light above us. He is still ordered to hover at that height and wait for instructions. Sorry about that. I'll try double check the rules better.))

Leelan will lead then horses beyond the entrance towards the tree on the right side. He will then take up a position by the cave entrance on the outside.

Aubrey will seek cover and hide behind the westernmost dark green tree.
Stealth if needed: 20 Perception if needed: 13

Guilim will move to close to where he can actually be in range of attacking anything that moves. He will move stick close to the light though due to his limited vision. 

The minutes feel like hours and the waiting is almost unbearable.  You are sure 10 minutes have passed but there is no noise coming from the cave and any sounds of combat from above are masked by the everpresenting sound of the rain, wind and rumbling thunder.

Another 5 minutes pass.

Then another.

Then there is a flash of orange light and a roar from deep inside the cave.

Soon there is the sound of shouting, and the clanging of steel.

Another light, this time the orange glow of flame, but without the flash or roar.  Now you can hear voices.  Far at first, but drawing closer.

A woman: "The horses!"

A man:  "Sixteen, you have to fly!  Find the nest!  Your mother depends on you!"

A man:  "Hold him back, Bracken!  Use the spectres!"

Luogaszil DEX: 23

Mieniera DEX:  9

Skeleton (+) Dex:  10

Skeleton DEX:  9

And then five figures burst forth into the rainy night.  The first is... some sort of owl-man?  It flies just under the netting and out of the cave mouth, strugging to gain altitude in the first wind and rain, nearly crashing into a small pine tree.  Next are two robed figures, a man and a woman.  They shimmer faintly in the darkness as though their very forms are enchanted somehow.  They are looking back over their shoulders into the cave and don't see the ambush, or the hazardous ground.  They skid across the ball bearings laid out by Grugga but they both manage to keep their feet.  On their heels are two skeletons... one normal size but the other a staggering 10 feet tall!  The first skeleton makes it past the bearings just fine, but they giant skeleton slips on the metal balls and falls prone on its back.

Initiative:

• Luogaszil  17
• Mieniera   10
• Skeleton+   18
• Skeleton  19
• Owl-man  3
• Grugga  21
• Guilim  2
• Leelan  22
• Rohuzar  15
• Aubrey  11

(You have the element of SURPRISE.  One free round to act before the enemy.  Once you have all taken a surprise round, we will enter initiative in the order rolled.  They white circle is the light from Grugga's axe.  Any characters without darkvision who target an enemy outside that circle do so at disadvantage.  The owl-man is presently 15ft above the ground and dashing away.)

Grugga yells, "heads up!" and tosses a crystalline grenade underhand about 20 feet in the air. At first it looks like he threw it way off target, but, as it reaches its apex, the hammer shatters the crystal and the strong wind blows a coating of shining purple dust right on the group of cultists. Faerie Fire Dex save DC 13. Affected cultists also begin to glow with a dim light.

The grey Orc then moves around the Halfling, hoping to avoid actively engaging the enemies just yet, and stands under where the owl-man is passing right over him. He raises his axe overhead in an attempt to shine light on both the creature in flight and the nearest cultists. ((I know the owl-thing is 15 feet up, so not sure if this is possible even with hand raised up.))

He then orders Zip to blast the owl thing, "Attack the one above me, Zip!" Zip's crystalline vision allows it to see the birdman in flight, even if its creator's light doesn't reach it.
Zip attack: 21
Zip damage: 6 Force
"Follow it!" Grugga orders. The small machine's stabilizers have trouble in this inclement weather, only allowing it to crawl forward 15 ft in the air. It remains 40 ft in altitude.

SURPRISE ROUND: 

Leelan steps out of hiding as the owl-person is flying away. Caught off guard by the rapid movement of the humanoid, the gnome quickly prepares an arrow. He mumbles a few arcane words and charges the arrow with a bit of magic. Once he’s ready, he will shoot at the owl-person.

Attack: 17 Damage: 8

Bonus action: Ensnaring Strike STR save DC 11 advantage if the creature counts as Large. On failure, is restrained for 1 min ad takes 1d6 piercing damage at the start of its turn. Can use an action to make a STR check against spell DC to break free.

Guilim is going to make way towards the edge of the light, raising his halberd and slashing at the larger skeleton from 10ft away.

Halberd: Attack: 26 Damage: 15

As he brings it down towards the undead thing he calls out and focuses on his foe, the blade of his weapon erupting with holy light as it strikes.

(So since it is undead that would make it 3d8 and then since I crit I believe that would make it 6d8 due to doubling the die. If I am wrong then let me know and O will reroll it as 3d8.)

Smite: 24

(Mid-Surprise Round Update)

The four non-flying enemies catch a face full of faerie fire from Grugga as they emerge from the cave.

Luogaszil DEX:  23

Mieniera DEX:  9

Skeleton+: DEX:  16

Skeleton DEX:  5

The female necromancer and the smaller skeleton are both covered with the dust and begin to glow with an unearthly pink shimmer (advantage on rolls to attack them).

The attacks from both Zip and Leelan catch the owl-man in mid-flight.  A cluster of vines erupts magically around the flying figure and it tries desperately to free itself and maintain its flight: STR 9.  It fails, and the plant material wraps it up, including its wings.  Unable to fly while restrained, it plummets to the earth taking another 6 bludgeoning damage and lands prone.

Guilim strides toward the giant skeleton which had tripped over the ball bearings.  He brings his mighty halberd down on the undead horror and there is a flash of magical energy.  The power animating the creature is not only broken, but the bones shrink back to a more normal size and scatter forcefully, sailing off in all directions, some back into the cave.  In the aftermath, there is just some tattered leather armor and a shortsword lying where the giant skeleton had been.

Rohuzar speaks to the air: “oh Air and dark guide my steps,” in a ritualistic manner, casting zephyr strike

He moves swiftly to the first necromancer and raises his battleaxe to try to split this one like a log (advantage for zephyr strike)

Attack: 26 Damage: Unable to parse dice roll. 

Seeing Leelan having effectively stopped the flight of the owl man Aubrey turns his attention to the glowing necromancer (Mieniera) swiftly cursing her (Hex, dex) before unleashing a golden beam of crackling force against her. 
Eldtritch blast: 11 Force: 12 Necrotic: 5
He then seeks full cover behind the closest tree if possible

Quote from nschrock >>

Rohuzar speaks to the air: “oh Air and dark guide my steps,” in a ritualistic manner, casting zephyr strike

He moves swiftly to the first necromancer and raises his battleaxe to try to split this one like a log (advantage for zephyr strike)

Attack: 18 Damage: 10 

 Forgot d8 force from spell 7 

Rohuzar moves like a blur, closing on the male necromancer - an elf, you see now - and delivers a swift blow with his axe across his left arm and shoulder, breaking through the magical protection around him and drawing blood through his robes (you can roll the extra 1d8 for force damage... even an 8 won't be quite enough to bring him down so it won't impact what happens next).

Aubrey fires off a golden beam of force at the female necromancer, but somehow even with her magical illumination he misses her in the dark.  He then retreats to cover behind one of the larger pine trees.

(Beginning of Non-Surprise Action)

The elf necromancer catches his breath and surveys the ambush he has charged into.  He locks onto the owl-man, collapsed and ensnared on the ground.  He hisses, "Protect Sixteen!  He must escape!  Burn these miserable wretches!"  He utters an incantation and begins to glow with a faint blue light, having cast haste upon himself.  With his extra action he takes the disengage action and moves out of Rohuzar's reach and to the side of the owl-man.

The owl-man struggles against the vines, taking 6 piercing damage and trying to break free... STR 12 ... but fails.  He remains trapped and prone on ground.

• Luogoszil
• Owl-Man
• Aubrey
• Skeleton
• Guilim/Leelan
• Leelan/Guilim
• Rohuzar
• Grugga
• Mieniera