Maerwyn finds her unable to make a weapon attack against the wounded Illithid since the cracked wall has been barring her from facing the Illithid in close proximity. If the barrier still stands, she'll be casting Misty Step in order to teleport herself through the wall.

OOC why cant she just misty step through now as bonus then use action to attack?

(According to the list of combat initiatives, it's not Maerwyn's turn to attack. If someone else or Magron can destroy the wall, she might not need to cast the spell.)

OOC: It's Maerwyn's turn. Post #390 confirms. After Maerwyn it is the Illithids turn. Might be better to just misty step then attack - as we don't know what the illithid will do on its turn, so downing it before it can take it is better. Otherwise, you've basically wasted your turn. The crack in the wall is large enough to see through - as this is how Grom attacked through it, Dusk teleported through it and how his elementals went through. I mean, if you would prefer Maerwyn to do nothing for her turn you can certainly do that, your choice. It just seems to me, though, you're having difficulty following turn order - which is understandable. So in case it helps:

ROUND 1

TURN 1: Wispy : went through crack in wall and attacked an Illithid.

TURN 2: Dusk - readied an action to switch places with Fire Spirit once Fire Spirit was through the wall.

TURN 3: Fire Spirit - went through crack in wall, Dusk took readied action to switch places, Fire Spirit went back through wall again and attacked Illithid.

TURN 4: Grom - peered through the crack in wall to Chain Lightning all 3 Illithids. Maerwyn uses reaction on one Illithid that died making it attack another, which also died. 1 Illithid remains.

TURN 5: Magron [Taken Out of Order] : hammed at the wall increasing the size of cracks.

[Current Turn] TURN 6: Maerwyn

[Next Turn] TURN 7: Illithids, if alive.

END OF ROUND 1

-

The next round if there is one will most likely follow this order:

Wispy
Magron
Dusk
Fire Spirit
Grom
Maerwyn
Illithid.

-

Hope that helps. But yes it is definitely Maerwyn's turn who gets her reaction back, too. She has an action, bonus action and movement she can take. Up to you what she does, just wanted to make sure there's no confusion.

(Maerwyn already used her turn by using the Blood Maledict feature as a reaction, the Illithid attacked a fellow Illithid in response to the feature. It's not Maerwyn's turn yet. See post #383 and let our DM Orange20 decide it.)

Reactions are taken on others turn, not your own. Its still your turn. 

(For your kind information, I already decided to take the ready action and I'll try to portray my PC according to my own wish, no powergaming please. I don't want to take suggestions and Maerwyn will not cast Misty Step, she will prepare to cast it. I think I have made myself clear to everyone. See post #391)

OOC as stated you can do as you want on your turn, that's fine. It's just you are incorrectly stating it is not Maerwyn's turn when the DM has already confirmed that it is. 

Should we then assume for your turn that Maerwyn does nothing (since you cannot ready bonus actions, unless DM choose to houserule otherwise)? 

We're not trying to tell you what to do. We're just confused because what you're putting doesn't make sense, and since it IS Maerwyn's turn, we cannot proceed until we know for sure. Again, to confirm, we're not trying to upset you - we're just trying to understand.

That's fine. I just wanted to make sure you're aware of how actions and reactions work, no offense meant.

Also, this: you can't normally Ready a spell which is cast with a Bonus Action, only one which uses an Action: "To be readied, a spell must have a casting time of 1 action..." Assuming the DM allows that, though, Sage Advice is that when you Ready an action to cast a spell, if you don't cast it before the start of your next turn it counts as cast and the spell slot is wasted. 

*I'll allow it since it isn't readying a spell longer than an action or bonus action. I hope everyone is fine with this ruling.*

On the mind flayer's turn:

The wounded illithid sends a telepathic message to Grom: If you don't kill me, I'll tell you help you and your group through our lair.

"Why don't you fight back? None of you fight back! If you don't want to fight, take a one way trip to the Astral Plane!"

Quote from ikarbino >>

"Why don't you fight back? None of you fight back! If you don't want to fight, take a one way trip to the Astral Plane!"

 The Astral isn't that hard. The illithd casts plane shift.

*I will be out for part of tomorrow.*

Dusk watches the Illithid vanish instead of retaliating. "Y'know, if it wasn't for the whole feeding on people's brains thing, I almost feel bad killing them. They seemed welcoming, they made us tea, and don't even try to hurt us when we murder them. 

If out of combat, Dusk will look around the room to see if he find clues or anything of particular interest. Investigation: 14

Maerwyn has started to brand the wounded Illithid using the Brand of Castigation feature. She doesn't want the creature to run away from this place, thus she's been trying to prevent the planar shift.

As a result, the branded creature can’t take the Dash action, and if a creature branded by her attempts to teleport or leave their current plane via ability, spell, or portal, it takes 14 psychic damage and must make a Wisdom saving throw. On a failure, the teleport or plane shift fails.

OOC - I apologise for bringing this up, but to be fair: you can't use Brand of Castigation (or Tethering) in this scenario. To brand a target you need to deal your Crimson Rite damage to them. You had the opportunity to misty step inside and attack/brand them - but you didn't and even made a fuss about deliberately choosing to not do so. So, none of the Illithids in that room got branded and the one that is plane shifted has not been hit by you, branded by you or affected by you in any way. 

So unless the DM changes how your class features work or Grom decides to Counterspell (seems unlikely given Grim told the Illithid to go away to the astral plane and it actually obliged), the mindflayer's escape is going to be successful. Dusk would have countered - but he has no Reaction yet, having used it to use Benign Transposition during Fire Spirit's turn.

(So, will this work if Maerwyn casts Misty Step as a bonus action, then attack the Illithid with the Scimitar when Grom is attempting to converse with the Illithid?)

Seeing the mind flayer dissappear Magron continues to wack the wall till there is a hole big enough for the party to move through.

Quote from InkedBee >>

(So, will this work if Maerwyn casts Misty Step as a bonus action, then attack the Illithid with the Scimitar when Grom is attempting to converse with the Illithid?)

As a reaction? No. Maybe the DM will let you retcon your turn (which we had taken to assume you just readied Misty Step) and we can say you had Misty Stepped and attacked/branded that Illithid, instead?