Chill nods to Rhogar as he quickly draws his rapier and thrusts at the creature as it attacks! 

"I'm with you Rhogar,  I'm getting out of the way of that bug blue whatever it is!"

He stabs with his rapier and then (BA) disengages and moves away 40 feet.

Attack: 23 Damage: 24

Con Save(DC 15) or become incapacitated until the start of Rhogar's next turn. 7

Sorry, that should be with advantage, second roll with +4 20

Chill (1-2), Kazri (3-4),  Krom (5-6), Rhogar (7-8) 7

Bite Attack: 11 Damage: 11 piercing

Claw Attack: 18 Damage: 12 slashing

Claw Attack: 16 Damage: 13 slashing

If a claw attack hits target must succeed on a DC 15 Constitution saving throw or be infected with a disease called chaos phage. While infected, the target can’t regain hit points, and its hit point maximum is reduced by attack 1 6 attack 2 8

Chill is out of range so I have to reroll who the creature attacks - Kazri (1-2),  Krom (3-4), Rhogar (5-6) 3

Krom Con save 30

The creature turns its fury against Krom and one of the claws catch an open spot in his armor.

Krom is up followed by Rhogar, Chill, Kazri in that order

Rhogar (81/87), Chill (73/73)+3, Kazri (68/114), Slaad (61/123), Krom (81/104), 

Gorrah (22/27), Oli (27/42)

(Anyone within 10ft adds +3 to saving throws, and get spell resistance)

"By Moradin's flaming beard, what is that creature?!" says Krom as the blue frog creature emerges from the Observatory. 

"Avoid its claws! They might drain your life force!" warns the dwarf after being hit by one of the razor-sharp claws. Krom, chanting an hymn of wounding swings his flaming hammer in retaliation. The flames around the hammer's head grows darker as the dwarf concludes his chant. "Let's finish this!"

Inflict wounds at 1st level 
Attack: 17 Damage: 17

Seeing the creature nearly succumb to his breath weapon only to shrug it off immediately sets Rhogar to growling something in draconic in clear frustration. "I know it not! But clearly it's no natural thing!" Said the dragonborn as he pulled into his free hand the bell branch wand and released a portion of its magic. Not to determine the creatures nature, for such was his self-assuredness in the moment, but instead conjure a viel over himself and Krom for at least a measure of added protection. 

Gorrah, in the meantime, continued to circle about above head and await her moment to strike.  

Action: Expend a charge of Bell Branch and Twinned(1sp) cast Protection from Evil and Good on self and Krom. 

Kazri will imbue her sword with divine magic again and strike at the creature.

(BA Divine Smite 3lvl 14)

Attack1: 26 Damage: 9

Attack2: 14 Damage: 10

Chill steadies his aim and fires at the creature with his crossbow and dropping his rapier to the ground.

Attack: 22 Damage: 28

Reroll piercing Damage per Piercer feat.  Either a 1d8 or 1d6 as I can't see the rolls (on mobile) but one of them is bad

d8 3

Or

d6 1

The concentrated efforts of the group take down the blue slaad and finish it off. 

Combat over.

The open doorway of the tower is before you. Nothing else is seen, heard or stirs within that you can tell. 

"Do you think that it has been imprisoned there from before the fall or it was a suprise left behind by the drow?" says Krom after the group has defeated the creature. "I guess that we will never know for sure."

"Before we head inside, gather around me for a few minutes." suggests the dwarf in a serious tone while gesturing at the group "We have all been wounded in the last two fights. Allow me to patch us a little bit before we move forward. Who knows what other surprises might be waiting inside the tower." 

As the group gathers, with a rhythmic tone, Krom begins chanting an hymn of healing. The dwarf's voice is steady and calming. Moradin's symbol pulses with a warm light, and gradually, the All-Father's fire starts to mend some of the group's wounds.

Prayer of healing The whole group (Oli and Gorrah included heal 9

Chill again picks up his rapier and rejoins the others.

"Big ugly blue brute.  Too many things left alive in this place. But the drow?  I don't know enough about them.   Who knows what strange things they have brought with them up from the underdark."

He makes sure the others are close enough to Krom for his healing as he doesn't need it himself. 

"Let's check out the tower." He says as he moves to the doorway and then stops and listens.

Perception 13

Chill looks within and hears nothing.  The bottom tower floor is littered with garbage and destroyed furniture. A staircase winds along the outer wall up and out of sight.  

“It is a good point Master Krom. Let us proceed with caution.” Kazri moves slowly to the upper floor. Shield in front. 

Kazri leads the way up the stairs.

When you reach the top you see a large telescope dominates a thirty-foot-diameter circular chamber at the top of the tower. Gazing into the telescope’s eyepiece is a hulking, frog-like biped in wizard’s robes that pays you no mind as it mumbles to itself.

Adamantine shields cover the observatory’s windows. A large chunk of stone from another building has crashed through the roof and embedded itself in the floor like a jagged stone knife. This intruding fragment bears a strange inscription on one side.

In a lower voice. “Hummm… Mr. Nimbatuul, I believe there is something for you to see up here.” Kazri gives the robed creature some personal space, but her sword is ready for any sign of trouble.