Quote from BaconExpert982 >>

((Does reckless attack include AOO?))

 Yes. You kill a quasit on its way out the door.

And with a nat 20, Quentin can arrive in time for this round. ^^ Feel free to attack.

OOC DM Question: How many could I hit with a 15 foot cone without hurting my friends or the prince?

((Well, you could probably not hit the three visible ones who just bit your friends.....there's a chance you might hit some invisible ones if you aim beyond them into the building.))

OOC: I don’t necessarily know the visible ones are there though so let’s just go with what I see. 

Reacting quickly Quentin already has his spell halfway cast and three bolts of arcane energy and slamming into the demon attack the Prince. 

Magic Missile: 

Dart 1: 5 

Dart 2:  5 

Dart 3:  5 

(( Elwin had already rolled a 12 to hit on an Opportunity Attack; I don't think it hits, but just in case... ))

Nope, Elwin just misses.

But Quentin takes down one quasit and wounds another.

So, to summarize the situation,  Mindartis and Elwin are still inside the open door of the summoning room with no visible quasits currently in melee range, while the faculty wizards fight the big demon and new quasits keep appearing.

Raven, Del, Quentin and Lintelér are in the hall, just inside the outer door. (Did you close the door behind you when you came in?)

It's the party's turn--Quentin has taken his action already.

CON 22 

And calling out Impertanent dismal-dreaming hedge-pig casts Mind Sliver directed at the one that bit him DC15 INT save or 4 psychic and -1d4 next save

((that tool tip is supposed to be Mind Sliver))

Seeing all here and an oncoming swarm, Del closes the outer door and sets his back to it for protection Come and shut that inner door - we can trap these in the hallway

Quasit saving throw: 6 The quasit clutches its little head in mental distress. Both visible quasits in the hallway are wounded now.

Prince Lintelér jabs at one with his sword: Attack: 14 Damage: 10

Only one visible hallway quasit left.

MOVEMENT: Elwin closes with the demon; he'll take whatever opportunity attacks this incurs.
BONUS ACTION: Elwin uses Hexblade's Curse on the demon; +3 to damage, and crit on 19/20.
ACTION: Two Attacks -
  1st Attack Roll: 24, Damage incl. crit: 9
   2nd Attack Roll: 15, Damage incl. crit: 11

Demon and quasits still takes 15 cold damage if they hit Elwin with a melee attack, until his frost armor is gone (currently 11 HP left.)

"Useless, useless, useless!" Elwin chided himself. He couldn't see them, had yet to strike one, and still there seemed like more just kept appearing. He turned toward the wizards. They weren't operating fast enough. "But you'll just get in the way!--" part of his brain screamed before he chose to ignore it, a different part yelling back "Then they'd better adapt!"

He ran straight toward the demon, muttering a curse along the way.

"Filth!" he finally roared as he closed the distance and lunged, fully intending on drawing the creature's attention. The thrust was quickly followed by a rapid withdraw-and-repeat, before Elwin spun into a defensive stance, snarling at the demon.

Raven Con save ... 12

Raven then attacks the nearest demon...

primary hand: 14

Damage: 6 

off-hand: 24

Damage: 1

sneak attack damage: 6

Raven neatly dispatches the bat-demon in her face! There are no visible quasits left in the hallway.

(Mindartis still has an action)

(Can i make it to the demon the wizards are fighting without dashing with 40 movement speed?)

((Yeah, it's a pretty big demon, and it's in the middle of the room, surrounded by wizards.))

Mindartis rushes to where the demon the wizards are fighting and takes two swipes at it recklessly.

Greatsword Attack 1: 13 Damage (radiant): 12

Greatsword Attack 2: 26 Damage (radiant): 12

Attacks on Mindartis are at advantage till the start of his next turn

Between the wizards' spells and the new vicious attacks form Elwin and Mindartis, the Nalfeshnee is looking pretty ragged. It saves vs. the last Banishment spell that Lonvari has thrown at it...: 5

With a pop, the big demon is gone.  The remaining visible quasits turn invisible, though no new ones seem to be arriving.

"Get out the door, and we'll fireball the place from the hallway! It's the only way to make sure!" shouts Mothander.  The wizards start casting Faerie Fire all around, and you can see that there are about twenty Quasits still sneaking around the room, flying around, or crawling on the ceiling.

One wizard steps into the hall, and after a few castings of Faerie Fire, those in the hallway can see six invisible forms trying to figure out how to open the exit door.

((The party is up: in the confusion of the banishment, there were no attacks on you this round))

Mindartis dashes out of the room, taking the wizards advice at face value and really not wanting to be the victim of fireball death.

Del draws his shortsword in case one moves by him for an AoO, but mainly he stays away (he will not retreat if that would provoke one on himself, obvs) and send another Mind Sliver Lily-Livered Pidgeon Egg! at Quasit number 1: DC15 INT save or 7 psychic and minus 1d4 next save

then he asks Is anyone in dire straits? ready to cast Healing Word as a bonus action

"Just cleanup duty left then..." Raven says as she moves to attack one of the glowing figures trying to open the door.

To hit: 13

Damage: 6

To hit: 25

Damage: 4

Sneak attack if appropriate: 7 (if under faerie fire then should be ok...)

“I’ll help the wizards with some cleanup.” Quentin says as he holds his wand towards his throat and mutters some arcane words. Assuming the exit door is clear of anyone but the Quasit Quentin is going to release a blast of fire from his mouth on to the Quasits in a 15 foot cone.

DC 14 Dex ST Half on Success: 11 Fire Damage