"Yes, let's not linger, we've been trudging long enough through these sewers." Damian says while meticulously cleaning up his boots before heading into battle again. He suggests Mortlock should take rear guard. "Let's recruit those servants of Myrkul first." He adds, ready to follow Rhogarr's lead.
Stealth: 6 Perception: 5

Since this sounds to be a major battle they are headed for, Remi expends a second hit die to come to full health.  He will need to have a full rest before he can recover anymore.

Hit die : 7, plus additional healing from Samuel's song : 1 

Rested and recovered, Remi heads back to the altar leading the way.  

Stealth : 21 

Perception : 24 

All - Assuming the rest of you aren't attempting Stealth Checks, I will move the narrative along.

If you all are willing to grant him amnesty and allow him to leave when your task is complete, Mortlock shares the following with you all during the short rest:

• • “I was betrayed. These assassins conspired with my brothers to kill me. If you hadn’t come along, I’d be dead. I owe you my life.”
  • “My family is paying the Dead Three cultists to murder people in the city. Our goal is to prove that the Flaming Fist can’t do its job. Once the city stops paying them, the Flaming Fist will have less of a reason to stay in Baldur’s Gate. With them gone, no one can stand in the way of my mother becoming the city’s next grand duke. If she gets her way, Baldur’s Gate will share Elturel’s fate and get dragged down into the Nine Hells.”
  • “My mother is one of the three remaining members of the Council of Four, which governs Baldur’s Gate. She was instrumental in convincing Ulder Ravengard to travel to Elturel and meet with its high overseer, Thavius Kreeg. With Ravengard gone, the Flaming Fist is leaderless and vulnerable.”
  • “The Dead Three cultists receive regular payments from my brother, Amrik, who runs his own moneylending business out of a tavern called the Low Lantern. My mother doesn’t expect much of me, but she thinks the world of Amrik. He’s always been her favorite.”
  • “My oldest brother, Thurstwell, uses imps as spies throughout the city. He has the bathhouse under surveillance and probably knows you’re here. He’s very sickly and seldom leaves our mother’s estate.”
  • “If you’ve made it this far, you’ve killed most of the leaders of the Dead Three cult. Without them, the cult will break up. The cultists keep their treasure nearby.”

Damian - Mortlock nods and waits for you all to filter out of the small room and into the hall before ducking through the doorway and assuming his position at the rear. Your meticulously cleaned boots are soon fouled once more as the party descends the stone steps at the western end of the hall into the ever present foul smelling water. As you tread upon the last submerged step you slip and slide down the steps on your rear into the water, tearing your shirt and creating a commotion. Mortlock reaches down and grabs you roughly by the arm and lifts you up, setting you back on your feet, "This isn't the place for supple riding boots."

Remi - For DM's eyes only:

You lead the way through a flooded portion of the dungeon, silent as a shadow on the dark water. You can't tell if it's paranoia, but you feel as if you're being watched from the depths of every shadowed corner. The rest of the party and Mortlock trudge along behind you, sounding remarkably like a herd of cattle fording a stream. You see the steps that lead south out of the water and toward the altar, to your left about 20ft away. What would you like to do? Please make another Stealth and Perception check if you wish to utilize those skills again.

All - What is the marching order?

Remi is walking toward the altar, he is looking around, feels like he’s being watched, he’s very paranoid currently.  

Stealth : 16  

Perception : 19  

"That was a lot of very important information Mortlock. I think Captain Zodge of the Flaming Fist would appreciate your testimony so that they can swiftly and harshly prosecute your family for treason. We'll see to it that they all will be united with their infernal masters within long, leaving you to preside over your family's stinking business. But I think you will need to expand on the part about Elturel being dragged down to the nine hells." Damian says, clearly intrigued by everything Mortlock just told them.

"Yes, thank you so much for pointing that out Mortlock, but I wasn't aware this morning that I would be wading through a SEWER all night, but I'm curious how you ended up on that island with that assassin, one may suspect you are easily duped Mortlock but surely that is not the case." Damian says dryly, trying to get his clothes in order before proceeding.
Stealth: 9 Perception: 11

Damian - Mortlock stops and narrows his eyes at you when you mention Captain Zodge, "That wasn't part of the deal. When the Death's Head is defeated I will seek passage on the next ship out of Baldur's Gate and never return. I care not what becomes of my family's stinking business, I've given you enough information to justify a formal investigation by that rabid dog Zodge. If you're thinking of reneging on our agreement, your corpses can feed the rats as well."

Remi - For DM's eyes only:

You follow behind Rhogarr as he wades through the water. Every shadow seems to writhe and shift in the flickering torchlight.

Initiative:

• Ankita: 9
• Damian: 19
• Samuel: 22
• Remi: 10
• Rhogarr: 11
• Mortlock: 11
• DHoB: 11

Initiative order:

1. Samuel
2. Damian
3. Mortlock
4. Rhogarr
5. DHoB
6. Remi
7. Ankita

All - A ghastly sight slips from shadows and reveals itself to Rhogarr. The fleshless face of a man, his lidless bloodshot eyes weeping blood and lipless mouth grinning madly. The Death's Head of Bhaal flicks its wrists and throws two daggers at Rhogarr before retreating northward.

Attack: 8 Damage: 8

Attack: 24 Damage: 9

The first fails to find a gap in his armor and falls to the water with a splash, but the second catches him just inside the collar of his armor.

Rhogarr DC 14 WIS Save: 9

The Death's Head of Bhaal's horrific visage causes Rhogarr to be stunned until the end of his next turn.

SURPRISE ROUND

Remi is up! The DHoB has already acted.

Samuel moves 2 squares west and one square north, behind the northwest pillar. He pulls his flute from his hip and removes a small piece of iron from the side. Samuel whispers some arcane words under his breath into the metal bar and a small, 1 inch arcane cage appears at the end as Samuel cast hold person at the Cultist. 
(Preposted)

Remi moves north and diagonal to the Death Head of Bhaal and slices at the cultist with his short sword.  Due to his fancy footwork he then moves away 10 feet without provoking an attack.  With his rakish audacity he applies his sneak attack damage too if he hits.  

Short sword :  Attack: 19 Damage: 9 

Dagger : Attack: 14 Damage: 4 

(to clarify movement, so that DHoB is one square away to the northeast, then he escapes away to the west 10 feet.)

Sneak attack damage : 10 

"I don't know what kind of deal you consider yourself having Mortlock but let me assure you that I can present much more attractive alternatives than being hunted for the rest of your life by the Flaming Fists and their allies around Faerûn as a traitor to Baldur's Gate. After we have taken care of this last cultist of Bhaal, I will help you get something in writing that will exonerate you of any crimes against Baldur's Gate commited by the rest of your traitorous family. You're a very important person now Mortlock, in fact you should consider your price for turning state witness, and after that you would still have your family's considerable fortune to shape whatever destiny you would want for yourself." Damian answers Mortlock just as they head into the ambush set by The Death's Head of Bhaal.

Remi - You're certain your shortsword hit the Death's Head of Bhaal, delivering a great wound, but in the flickering torchlight the wound is there one second and gone the next.

DHoB Unstoppable Charges (2/3)

Samuel - DHoB WIS Save: 7

The fearsome cultist is suddenly paralyzed by Samuel's spell-binding words.

Damian is up! Followed by Mortlock, then Rhogarr.

Will upload map in a bit.

Damian follows the last cultist and laughs evilly as he sees him rendered helpless by Sam's magic. "Your murdering nights are over cultist, prepare to meet your unforgiving master." He snarls and swiftly curses the cultist before sending a beam of hellish red crackling force towards him.

Movement: 3 squares to the west.
Bonus action: Hex Wis on cultist.
Action: Attack cultist Eldritch blast: 15 Force: 4 Necrotic: 4

Damian - The Eldritch Blast strikes true, pummeling the cultist in its gut.

Mortlock will stomp over to the cultist and raise his Greatclub high above his head.

Attack: 17 If hit, 16

Attack: 13 If hit, 16

He brings the greatclub down upon the immobilized cultist, bludgeoning him. His second swing glances off without dealing any damage.

Rhogarr is paralyzed until the end of his turn.

The Death's Head of Bhaal tries to break free of the magical paralysis. The magic holds and he is immobilized still!

WIS Save: 5

Unsurprisingly, the DHoB has Magic Resistance. The death’s head has advantage on saving throws against spells and other magical effects.

Remi is up! Followed by Ankita, then Samuel.

Damian - Prior to combat, Mortlock scratches his chin with one hand as he holds his greatclub over his shoulder with the other hand, "You speak sense... Can you truly do this?" He eyes your signet ring and obviously fine clothing, "I do not wish to spend my days looking over my shoulder nor wondering where my next meal will come from. There will always be a stain upon my family name, but perhaps in time I can at least show that I am not the monster I appear to be."

"Let me handle the interactions with the Flaming Fists for now Mortlock, you will need a good lawyer like me to get yourself a good deal. It might take a couple of days though so we must find you place to lie low while I handle negotiations, but I'm sure we will come up with something." Damian says confidently.

“The Flaming Fist sought to eradicate the cultist plaguing this city, a task in which all of us would testify you played a vital role. We shall discuss specifics at a more fitting time.” 
Persuasion: 27 (Minimum of 17 due to Silver Tongue) 

Damian nods at Sam's words. This one would be particularly useful, he thought.

Remi steps forward to one square west of the DHoB and strikes, then steps away to his current spot.  He takes advantage of the paralyzed cultist.  

Short sword attack :  13 , damage if hits : 10 

Dagger : 16 , damage if hits : 7 

Sneak attack damage : 10 

Remi - The immobile Death's Head is unable to stop your shortsword from skewering it.

Ankita is up! Followed by Samuel, then Damian.

Damian & Samuel - Mortlock nods and accepts your words as truth, trusting you for now.

Ankita hears the battle going on around the corner and has no idea what she’s running into, but around the corner she goes.  And in dismay she sees she cannot get to it from where she is with the water they are wading through.  In a fit of anger she emits an animal sounding howl and turns into a cave badger, snarling with her beady little eyes fixed on their enemy.