The entire party dashes for the entryway, for fear of being overwhelmed.  

From all the stairwells now, you hear the chittering of monstrous aberrations and the scraping of tentacles across stone walls and steps.  

Aubrey's spell still seals the upper left stair, and you hear renewed howls as another wave of Chokers races headlong into the hungry grasp of Hadar.

Meanwhile, four more Chokers appear at the upper right stair; four appear at the lower left stair; and seven appear at the lower right stair, climbing over each other and even up onto the walls to squeeze through the passageway.

Aubrey:

1. Party!
2. Chokers!

     Raqisat is still looking to move towards the entrance, hoping to get there before he is surrounded, but his higher priority is to stay with Grugga and the others nearest him... (Leelan is a bit south... Who is the X looking icon?)  As such Raqisat is looking to move South rather than into melee with anything but he does have his weapons out and is ready to defend,

Seeing the tentacled aberrations swarm out of the side towers now too, Aubrey is at least somewhat relieved by the brief report from his weird familiar. "There's no more of them coming now, just fall back to defensive positions and we''ll handle this." The young noble calls out to the team in an encouraging tone, then calling upon the powers of his talisman once more to deal with the mass of enemies swarming out of the eastern tower, opening a new dark gate there. He then continues to fall back to the entrance.

(Casting Hunger of Hadar centered on the southwestern choker of the group to the east, hopefully disorienting them too, then moving 30 feet south.)

As Aubrey redirects his powers to open a new [spell[hunger of hadar;dark gate[/spell] over the Chokers at the eastern stair, his prior rift dissipates and then winks out of existence, revealing six disoriented and acid-burned chokers on the upper left stairs.

[Aubrey, go ahead and roll 2d6 acid damage to see how badly those six are hurt.  Previously, as long as the spell was on them for their arrival + their turn, I was ruling the spell was killing them outright without a mechanic for them to escape.  Since these Chokers just arrived this round, they haven't felt the double-whammy of the spell yet.

It's still an effective trade.  You're swapping 7 close, unharmed Chokers for 6 further away, injured Chokers.]

The Chokers on the right staircase vanish as a sphere of eerie, infinite darkness envelopes them.

Grugga dashes towards the entrance and commands Zap to do the same. He nods at Raqisat and says, "Don't worry, there'll be plenty left for us!"

(Acidic tentacles from beyond: 6)

Cosette provides cover for Grugga and Raquisat's retreat as she continues to back up. She'd take aim and release a bolt.

Target: Prioritize any visible injured target. If none, closest visible threat.
Attack: 18 Damage: 6

Now standing at a new, more defensive position, Leelan assesses the threats around the party. Aubrey seems to be handling the creatures on the eastern side with his unnerving spell. The western side is a more viable option.

Leelan focuses on one of the aberrations and fires two shots at it.

Bonus action: Slayer’s Prey

Action: 

Attack 1: 25 Damage: 4

Attack 2: 21 Damage: 9

Slayer’s prey bonus damage on first hit: 6

As most of the party continues to move south to the exit, Leelan stands his ground, releasing a pair of arrows at one of the Chokers emerging from the west side stairs.  Though his aim felt good, neither arrow is able to penetrate the odd, rubbery hide of these aberrations.  

Raqisat remains back with Leelan, trying to stay between him and the horde of little monsters.

To the west, the four Chokers dash toward Leelan and Raqisat...

Aberrant Quickness (Recharges after a Short or Long Rest). The choker can take an extra action on its turn.

... And they just keep coming!  Closing quickly to within 10', the Chokers reach out with their starfish-like suckered paws, trying to grasp Raqisat and Leelan.

Choker 1 (Attacking Raqisat)

• Tentacle Attack: 24 Piercing Damage: 6  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.
• Tentacle Attack: 6 Piercing Damage: 6  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.

Choker 2 (Attacking Raqisat)

• Tentacle Attack: 20 Piercing Damage: 6  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.
• Tentacle Attack: 12 Piercing Damage: 6  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.

Choker 3 (Attacking Leelan)

• Tentacle Attack: 21 Piercing Damage: 4  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.
• Tentacle Attack: 7 Piercing Damage: 5  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.

Choker 4 (Attacking Leelan)

• Tentacle Attack: 18 Piercing Damage: 7  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.
• Tentacle Attack: 21 Piercing Damage: 6  If the target is a Large or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the choker can’t use this tentacle on another target. The choker has two tentacles. If this attack is a critical hit, the target also can’t breathe or speak until the grapple ends.

To the North, the Chokers just released from the Hunger of Hadar are stunned and don't take any action this round.  The Chokers on the NE stair, however, race South, their spider like limbs holding their scrawny bodies just off the floor as they scrape and scrabble across the sandy chamber toward Leelan and Raqisat.   

Aberrant Quickness (Recharges after a Short or Long Rest). The choker can take an extra action on its turn.

They surge unnaturally fast, coming to a stop just 10' away from Leelan and Raqisat.

The party is back up!  And Raq and Leelan are surrounded!

Map with engaged grapple lines...

The young noble watches helplessly as Leelan and Raqisat are swarmed by the impossibly quick tentacled aberrations. They would only last moments now and the were more aberrations coming. Aubrey grits his teeth, hoping he and the others could blast a path open for those caught to retreat, sending two golden beams of eldritch crackling force streaking towards the closest enemy.

Eldritch Blast: 12 Force: 6
Eldritch Blast: 16 Force: 1

(Sorry, missed +4 on the damage rolls)

Choker 1 vs Raqisat...
     As the choker moves into range with Ragisat (10' with whip) Raqisat will spend a Superiority Die to use Brace and make an attack against it:
Raqisat Radiant Whip vs Choker:  17 to hit for 8 Slashing Damage +3 Damage from Brace plus 6 sneak attack damage as this comes before Raq is grappled... 17 Dam total.

Choker Attacks
     24 hits, 6 damage taken.  Raqisat is grappled and restrained.
     6 misses so no damage is taken.  Except creature has Advantage due to Raqisat being restrained.  Rolling for that:  22
          If that now hits (I'll find out after I post) then I will take the damage.  If it misses then Raqisat will Riposte and use his reaction to attack it:
Raqisat Radiant Whip vs Choker Reaction Attack:  24 to hit for 8 Slashing Damage +2 Damage from Riposte...  Actually this would be at disadvantage due to being grappled and restrained and Raqisat is actually chosing not to waste the Superiority Die at this time... Plus Brace already used my reaction anyway.

---

Choker 2 vs Raqisat
     20 hits, 6 damage taken. Raqisat still grappled and restrained...  This would be at advantage due to grappled & restrained so rolling for that just in case of Nat 20:  9
     12 misses so no damage is taken but once again this would be at Advantage due to grappled & restrained. Rolling for that: 24
          If that now hits then I will take damage...

---

     On his own turn Raqisat curses colorfully and swallows loudly.  "This is not good," he says in attempt to get understatement of the year and considers attempting to break the grapple but realizes there is no point - He's grappled by at least two tentacles and breaking free of one would not improve his situation at all in regards to counter attacking.  Instead he just attacks the best he can versus Choker 1.

Radiant Whip vs Choker 1: (15, 17)+7 for 22 to hit and 8 slashing damage.

Bonus Action Scimitar vs Choker 1: (10, 12)+6 for 16 to hit and 7 slashing damage.

...edited to try to kickstart the Dice Rolls Not Yet Available nonsense...

((Looks like the 2 misses become hits so 12 more damage taken...))

[Thanks for updating my math... but wait!  There's more!

I completely forgot that Raqisat was under protection from evil and good cast by Leelan.  So, any attacks against him would be at disadvantage, until one succeeded and grappled/restrained him, at which point those would cancel out and the rest of the attacks would be straight rolls as the advantage/disadvantage cancel out.  

Furthermore, Raqisat's readied attack kills the first choker, so those attacks never take place and the damage from Choker 1 never occurs.  

The first 'real' attack would be the first attack from Choker 2... and it should be at disadvantage so here's another roll... 19.  So that attack still hits and triggers the grappled/restrained condition.  Same 6 damage as before.

The second attack from Choker 2 occurs, but it is a straight attack (as rolled originally, since the magic spell and the restrained condition cancel out).  It is still a 12 and still misses.  

So... only one of the Chokers attacking Raq is still alive, Raq is grappled/restrained but, for the moment, still protected by Leelan's spell...

...

...

Which brings us to what I got wrong with Leelan.  

Every attack after the first, which hit, should have been at advantage since Leelan was grappled/restrained by the first attack.  Two hit anyways, but there is always the chance of a crit.  So three more attack rolls... 17, 19, 22.

That 17 turns the missed second attack into another hit, for 5 damage.  Leelan is now grapped/restrained by all 4 tentacles on the two Chokers attacking him.

He also needs to make 4 Concentration checks to keep up his spell on Raqisat.

Whew!]

Concentration checks:

1st: 16

2nd: 17

3rd: 7

4th: 5

Concentration still intact!

Grugga turns his head to check on Raqisat and stops in his tracks when he doesn't see him. "Shit!" he says, turning around and seeing the creatures converging on Leelan and Raq. He immediately turns heel and rushes in to join the fray.

Movement: 30' straight north

Action: attack the only creature in range (the one grappling Leelan twice, by the looks of it). Attacking 1h as usual to keep one hand free for shield.

Attack 1: 22 to hit, 9 magical slashing
Attack 2: 25 to hit, 13 magical slashing

Bonus action: order Zap to dash inbetween Raq and Leelan on the north side - on the side of the creatures who haven't attacked yet.

Grugga careens in at full force, sword held high in one hand. Right as the closest creature grabs Leelan with its weird, stretchy tentacle hands, Grugga arrives and attempts to cleave the arms right off its body with a powerful stroke, then aims for its head with another. Meanwhile, he whistle in a 'language' only he and his magical automaton know and the spinning top careens full tilt toward the group of creatures arriving from the north, interposing itself between them and Grugga's allies.

(Damage on Raq modified and should be correct now)