Xaul is glad to see the orc mage perish so quickly; now the brain is the only focus. He casts Frostbite again on it by getting a couple magical drops of water from his flask on his palm and flicking the freezing water at the brain. [Pretty sure you've got it by now but DC 12 Con Save or 4 cold damage and -10 speed] The tentacle then swashes like a wave through the room to strike down on the brain once more: 15 to hit, 5 cold damage on a hit.

Damros is startled by the speed of the charging brain on legs.   Before he can even react to the charging monster, he is blasted by the cone of fire from the magic wielding orc.

Dex Save: 10     Damros takes the full blast of fire and crumples to the ground, unmoving.

OOC: Yes, it was the players character that was killed and taking over by the walking brain. The player is fine and made a new character.

Dex save 13: The agile Kylaski steps back for a moment, just out range of the fire. 

Kylaski doesn't like the brain being right next to her, so she casts Dissonant Whispers at the brain. She whispers a discordant melody that only the brain can hear.

DC 13 Wisdom save or take 14 damage and immediately use its reaction, if available, to move as far as its speed allows away from Kylaski.

OOC Appropriate spell for someone who gets there power from madness...

Forgot to say, it's half damage on a success Kylaski.

I'll do an update round and the brains turn when it isn't the middle of the night 

It worked on the last guy, and that’s enough for her. The genie warlock draws her fingers together then releases them to point at the gooey brain, stabbing her thumbs into the air like she’s gouging eyes. 

“Dritch bas el!” she calls to summon her Eldritch Blast. 

(AR: 13. DR: 6 force and 2 Thunder)

So:

Damros goes unconscious. Death save (will roll it for time saving, but happy for you to roll them when others are doing actions): 10

Xaul causes frost to settle on the brain, and smacks it with a tentacle.

The brain then attempts to magically eat and replace Damros' brain (I felt very bad rolling for this as it would be instant, irreversible death) however it rolls a 1 (for a total of 2). Even if Damros rolls a 1, it would be a tie and Damros wins, thereby nothing happening.

Kylaski and Skye both simultaneously cast spells. It is hard to judge who was more effective, but either way, the brain dies. (both attacks were individually enough to do it. Skye's only just so)

Renaer pulls a healing potion (7 points of healing) out of his pocket and thrusts it to Kylaski "For him on the floor" and then goes running over to Floon who is barely conscious (1hp) and tries to help him stand.

We are out of initiative, so no new map. 

Xaul is glad to see the brain perish, and sees that the party is tending to the wounds of Damros and Floon. He avoids harvesting the fluids of the brain though; he's got no clue who it might have belonged to and besides, he'd rather get the blood of an illithid than its mediocre minion. Once the party is all stable and ready to discuss next moves, he proposes 'Should we head back from here now and return Floon to complete our mission, or head after that tentacly fellow? Chances are he may pose a threat to other citizens again later on, and we could always just see how far we can get through here before heading back, taking out as big a chunk as we can from this organisation to maximise how much of a difference we can make. We might even be able to gather some treasure if we're lucky!'

Damros spits and sputters as Kylaski  poured the healing potion into Damros' open mouth.  Taking a moment to do an internal inventory of his well being, Damros sits up to get his bearings.  "I take it we 'won'... or at least killed everything?"

Working slowly, Damros rooted about in his backpack until he found the well wrapped healing potion he carrys.  Popping the seal, he upends the vial and lets it drain down his throat.  (game log:  2d4+2 >>  6+2 = 8 pt healing)   

With a full body shiver, he finally feels himself again as he puts out a hand to Kylaski  for help to his feet.    Standing up with a wobble, he turns to his comrades, "Thanks to you all for NOT letting that brain replace mine.  Those disgusting pests...,"  he has a full body shiver at his near 'miss', "... those are just a few steps below having that mind flayer messing with your head -- literally."   

He looks around the room, "Speaking of -- where is the illithid's body?"

Damros (with the only one high enough passive perception) hears what sounds like maybe a portal coming from the doors the illithid went through.

Kylaski answers "Tentacle face left the room when we entered. Though it didn't look like he was scared of us."

Xaul is discussing with the party about their next moves and celebrating victory ,when they all hear an insidious hiss in Xaul's own voice emanating from his flask, saying 'Boy...release me...' Xaul's demeanor instantly switches up as he pours out his hip flask out onto the floor to create a large puddle that is curiously a perfect circle. Kneeling down like he is praying, he whispers 'Master, it has been long since you have addressed me from the Sunken Planes with your voice. I praise you, and ask what has led to this intervention.' The reflection looks back and shockingly, it moves on its own, hissing 'You have fought strongly with my magic, shown me lands I cannot reach and have fed me the blood of many magical beasts. Thus I feel it is fair that now is the time for me to impart more magic onto you.' Xaul is taken aback and says 'As you bid it Master, what do you impose?' The reflection simply states 'I have seen your arcane hand lacks use. It shall now ebb away, yet cascade into a new form, one that can punch forth and strike your enemies with strength and power.' Xaul nods, and says 'Thank you, O Humid Oasis.' The reflection continues on to say 'What is more, you deserve a new spell to represent your dabbling in the arts of fire. I do not disagree with this path, for it can be just as cleansing as water, so I imbue a spell that represents the flames of the deepest ocean trenches, and I hope you can cleanse your enemies with it well' It grins maliciously, and suddenly, Xaul explodes in bright, sea green flames. He utters a cry of shock, but not pain, and just as quickly as the flames appear they fade. The reflection watches this gladly and says 'Do not say I don't observe your doings. That spell should suit you nicely. Now, finally, we come to the reason I speak to you now. I feel you are now worthy to consume part of myself into you and begin to access the true potential of my arcane freedom. However, my boundless magic is to be used wisely, for it can have consequences if you misuse it.' Xaul nods in agreement again and says 'I understand.' The reflection nods back, whether or if it's own accord or just as reflections do, responding 'Drink a small portion of myself and swill it about your mouth. You shall find yourself able to interpret and speak to beasts. Next, pour a portion over your body. It shall change and bend to your liking, like the tides. However, these are both temporary, yet you may use them as you like. Now, I trust you will make good progress in your missions. It is time for my mind to sink back to the depths. We shall meet again soon' Then, the reflection goes silent once more, and the water, like it's in reverse, flows back into the flask.

[TL:DR, Xaul has swapped Mage Hand for Eldritch Blast and learnt the Hellish Rebuke spell. He has also got the Mask Of Many Faces and Beast Speech Invocations]

Damros skirts around Xaul and the puddle he was talking to -- to go over to the door indicated by Kylaski.  
With a quick listen  ( Perception: 18 ), he jerks the door open to see if the illithid is still there.

Skye leaves Xual and his watery appertif to their conversation and heads to Floon and Renaer. 

"You're Floon are you?" She offers out one hand. "I'm here to deliver you, postage pre-paid."

Kylaski ghost to stand behind Damros before he opens the door. That looks more interesting than Floon. Sehaethas would disagree, but he's not in control.

As you open the door you see another room with an open portal on the southern wall, and a pedestal in the middle of the room. There is doors opposite you, and to the north and a corridor to the northwest with steps up. As you decide what to do about the portal, it closes. There is no sign of the mindflayer.

Skye, Floon merely nods, and manages a weak smile at the package joke. He seems to exhausted and injured for much more. 

Damros will turn back to address his fellow adventurers, "Looks like the mind flayer just portal'ed out of here.  I have no idea where to... so that is the town's problem for now."

He waves a hand to the room behind him, "The portal shut down.  There is a pedestal in here if anyone feels lucky and more doors to the Gods Know Where.  Are we now Sewer Inspectors or do we go and claim our 'postage fee' for returning Mister Floon?  Me, I vote return Floon and collect our reward.  The sewer will still be here and who knows what the city watch will want done 'unseen' once they learn of what we saw.  Intellect Devourers and Illithid in the sewers?   I bet we might get a reward for warning them!"

Kylaski sounds disappointed as she says "I think squid face left through the portal. but maybe the stairs up give us a way out other than the stinky sewer."

Damros slaps Kylaski on the back. "Finally, you are earning your pay."  He actually semi smirks at Kylaski before stepping aside from the open doorway.

As they help the injured nobleman to his feet, Skye makes sure she is behind him as they begin to exit from the sewers. She asks him why they wanted him, if he knows, as they head back towards the first inn and Volo.

The pedestal is carved with a small symbol: a perfect circle with ten equidistant spokes radiating outward from its circumference. In the middle of this circle is a smaller circular indentation that bears a passing resemblance to a lidless eye.

Before you leave the room, I will be generous and assume someone will check the room and see that behind the chair the mindflayer was sat on is a small, unlocked wooden chest containing two potion of healing, 16 gp, 82 sp, and 250 cp. 

You go up the stairs (I won't bother updating the map) and it leads to a secret door (totally not secret from this side) that is still in the sewers, just south (and on the opposite side) of the one you entered the lair from. (and that is the door the bards found, totally bypassing the rest of this area as they then left after the boss fight too). You still have to go through the sewers but there is nothing else encountered in them. 

Floon once he is able to regain some strength will confirm to Skye and Renear what Renaear suspected. It was a case of mistaken identity where they thought they had managed to capture Renear.