There is a 3x5 window you can see through, but it points to the ground and gives a shallow view of the outside. From what you can see, there are at least several dozen lizardmen with a fraction  showing the shaman tattoos you've seen already. There are also body parts you can make in the crowd of other creatures: ogres, trolls, gnolls, etc. In front of them all is a monstrous lizardman in piecemeal plate armor bearing a shield and spear.

"It's a show of force. We can either face it with force or find out what they want. Which will it be?"

“I doubt that they’ll wait in the area for an hour while other enemies may be on the prowl. They’ll likely begin advancing further outwards to find us.”

//we should wait at least a little while. Too many to try and take on at once.//

The voice of this "Szazzul" seems to address his horde now, "Minions, you heard the call and hunger for blood. These intruders will be found and will serve as the first drops of a torent of carnage. But your Master knows your hunger must be sated, lest you turn on each other. Such it is, then, that I give you entertainment!" The massive lizardman at the head never once moved during this speech, but raised a cheer with the rest of the crowd.

From your vantage point, some movement can be seen within. After a minute, a dozen civilians - dwarves, humans, elves, and others - are pushed forward into the clearing as the throng begins to circle around them. A few take the initiative to rush where you fought the lizardmen and grab weapons. Others stand in horror at the impending doom that approaches.

Solomon’s eyes go wide as he mouths “Oh no...” He frantically writes, “We can’t just let them get slaughtered.”

//I think we should all drop at once and go for the leader and the big ones.

Is the speaker close enough to reach in one round?

Or better yet, ogres  or trolls...

You are not facing an army, per se. The total amount gathered is about 80 (40 lizardmen, 10 shaman, 5 lizardmen champions, Big Lizardman, 6 ogres, 3 trolls, 16 gnolls, and a patridge in a pear tree). Szazzul is not immediately visible, but his presence is apparent. Since some of the prisoners scrambled for weapons under your portal, the circle is generally centered around you as well.

The monstrous horde seems content to play with its prey for the moment, but the first death could easily incite chaos.

At what distance? Are there enemies within 30'of the portal?

OOC: I can make a forcefield for 10 minutes. That’s all I got for ideas...

If an ogre is within 30':

That critical adds 11

Initiative: 18

so that spoiler omitted some stuff between the 14 and 26.  Hopefully this clears some of that up:

...14 lightning (dex dc 13 for half) and the javelin target is attacked: 26 for 26...

Luckily, there is no attack roll for your Javelin. After the command, it turns into a lightning bolt until it hits the target (who is also subject to it), but there is no actual "attack", just the DC13 DEX save. I like easy math, especially in massed-combat, so I'll be rolling for 1/4 of the total number of each creature to be hit (plus the leader)

Leader: 4

12 lizardmen (10 of them Shaman): 18, 2, 3, 2, 10, 10, 12, 7, 2, 6, 8, 6

1 Ogre: 14

1 Gnoll: 20

Again, for the sake of easy math, I'm combining the hp of all the different creature types into a mass of hp. As you chew through them, the overall numbers will drop off also, because this is considered a confined space, only some of the enemies will be able to attack each round/turn while the party and prisoners will each get a full turn every round.

Initiative time! Party receives a surprise round due to, "How did people just fall out of thin air like that?!"

Leader: 11, 

Lizardmen: 5

Shaman: 4

Ogres: 4

Trolls: 19

Gnolls: 7

“Oh. Oh no. This is going to go horribly.” Solomon will slide down the rope, Batman-style, and will plant a bean. At this point, why not. He’ll also tell the prisoners, “Climb the rope!”

For good measure, Rex adds a superiority die to that crit, Goading Attack, add 4damage and needs a wis dc 17 or rolls attacks at disadvantage against creatures other than Rex.

Quote from Barlow >>

Luckily, there is no attack roll for your Javelin.

LAIR20, Ogres3(173)20, Rex18, Trolls3(252)17, Leader(89)6, Gnolls16(347)5, Lizardmen38(858)3, Shaman7(182)2

(So we're clear: It's Monster, Quantity, Total HP, Initiative)

Rex's surpise assault takes the crowd off-guard. Several lizardmen are fried by the lightning and an ogre nicked by the bolt is immediately felled by the blade. The crowd seems hesitant about charging a behemoth of a warrior that just threw lightning. That is, until a rumbling in the ground takes hold of Rex's attention...

Szazzul, The Great Shadow, stands before you, a gigantic black dragon towering over his minions. His face seems emaciated and drawn, creating a countenance like a laughing skull, "Well, well. It comes to play. And here I thought my minions would be bored quickly with my entertainment. Come now, you could not have dispatched a dozen so quickly, where are your friends?" The question is rhetorical as there is no intent for a conversation. The crowd surges to attack!