Two of Grandpa's blasts of flame hit and sizzle on the oily writhing mass that is loosing it's serpentine form now that it is no longer traveling forward, but turning to fight. The third one misses, as Black leaps forward, steel fangs savaging the one that BB just finished smashing with his hammer.

Larry takes advantage of the Robed One being temporarily blinded and unleashes a torrent of attacks in them (flat rolls due to blinded advantage canceling out with the disadvantage from skitterers).

Attack: 14 Damage: 13
Attack: 22 Damage: 9
Attack: 22 Damage: 11
Attack: 20 Damage: 10

His first swing misses, but the remaining three connect.

Four tendrils lash out from the Amorphous (formerly Serpentine) Thing, one each going for BB, Black, Grandpa and Brynn.

Tendril on BB: 20 to hit
Tendril on Black: 17 to hit
Tendril on Grandpa: 25 to hit
Tendril on Brynn: 26 to hit

((Gramps, the acid does damage on the start of its turn?))

Gramps: 18
Larry: 12
Amorphous Thing: 12 (-28 acided)  <-------<<
Bryn: 8 
Keller: 8
Robed One: 7 (-88) blind
BB: 5
Bolt: 2
Skitterers: 2 (X/X/-5 blind /-5 blind /X/X/? blind/? blind /? blind /?/?/? || X/X/X/-15/-15/-15/?/?/?/?/?/? )

((Think those all hit...))

The tendrils begin pulling back into the creature, dragging all four of you closer to its position, BB coming within range of a gaping orifice that splits open one side.

Bite on BB: Attack: 27 Damage: 28

Gramps: 18 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)
Larry: 12 (disadvantage on saves, disadvantage on attacks not vs a skitterer)
Amorphous Thing: 12 (-28 acided, 4/6 tendrils occupied)
Bryn: 8 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)   <-------<<
Keller: 8  <-------<<
Robed One: 7 (-88) blind
BB: 5 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer, enemies have advantage on attacks vs BB until skitterers turn)
Bolt: 2 ( disadvantage on saves, disadvantage on attacks not vs a skitterer)
Skitterers: 2 (X/X/-5 blind /-5 blind /X/X/? blind/? blind /? blind /?/?/? || X/X/X/-15/-15/-15/?/?/?/?/?/? )

((Does BB want to use another Lucky on that crit? Damage type is bludgeoning as it gums away on him, btw.))

((Yes please, another lucky while I remember them. Also ouch.))

((Nice maps! don't think you use talespire before indeed; Acid does indeed do damage at start of its turn))

Acid dmg: 16
for the attack on gramps Black would use his deflect attack to cause disadvantage btw

Lucky on BB's Crit: 15

Deflect Attack on to Hit Gramps: 24

BB is pulled within the slavering mouth, but manages to lean back just as it glomps closed inches from his face.

((Tendril still hits Gramps.))

((My thoughts for NPCs, Brynn misty step and attack (try to clear skit?), Keller unload on Robed One while it's blinded. Thoughts?))

Gramps: 18 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)
Larry: 12 (disadvantage on saves, disadvantage on attacks not vs a skitterer)
Amorphous Thing: 12 (-34 acided, 4/6 tendrils occupied)
Bryn: 8 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)   <-------<<
Keller: 8  <-------<<
Robed One: 7 (-88) blind
BB: 5 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer, enemies have advantage on attacks vs BB until skitterers turn)
Bolt: 2 ( disadvantage on saves, disadvantage on attacks not vs a skitterer)
Skitterers: 2 (X/X/-5 blind /-5 blind /X/X/? blind/? blind /? blind /?/?/? || X/X/X/-15/-15/-15/?/?/?/?/?/? )

((Sounds good))

((Sorry for being off for so long, I have been very sick.  Anyway, with Bolt's 16AC I don't think any of the three attacks I see after I rolled the saves makes it, but they also show as 'potentially manipulated' and none show 6 damage, so I'm just gonna assume you're right and apply the 6 damage to her))

((I think the NPC suggestions are solid, use that blindness and get rid of the Robed One.))

Bryn vanishes in a puff of mist, reappearing on the slope before climbing a bit further up and sending a crossbow bolt down to try to clear some of the skittering underlings from around Grandpa and BB. He calls upon the tides of chaos to guide his shot.

Keller, seeing the Robed One blinded, unleashes magic and conjured blades, sending a tidal warbling mass of chaos at the foe and following it up with a hurled dagger.

(Tides of Chaos on a 2nd level Chaos bolt, quickened to a bonus action, psychic blade with action.))

((Rolls weren't showing, so clipped them out and retrying here.))

Crossbow Attack: 14 Damage: 7

Chaos Bolt Attack: 21 Damage: 16

Psychic Blade Attack: 22 Damage: 9 + 2 sneak on hit

((Forgot Keller doesn't need to use Tides, so scratch that but should also have adv on psychic blade, so checking for crit: 14 I also can't see if the results for the 2d8, so will reroll separate here for chaos bolt purposes. 4 and 3

No longer blinded, but severely injured, the Robed One hisses in a moistly sibilant tone, waving an arm and vanishing. Larry can hear the movement and see a disturbance in the dust, so lashes out at the retreating mage.

Attack: 10 Damage: 8

His swing goes wide.

The footsteps are surrounded by purple dust, then vanish. Larry is doubtful the caster is still in the last place he saw movement from it.

((Bryn dropped one of the skitterers, went for one of the ones injured by BB's thunderwave.))

Gramps: 18 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)
Larry: 12 (disadvantage on saves, disadvantage on attacks not vs a skitterer)
Amorphous Thing: 12 (-43 acided, 3/6 tendrils occupied)
Bryn: 8 
Keller: 8
Robed One: 7 (-115) (invisible, location unknown)
BB: 5 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer, enemies have advantage on attacks vs BB until skitterers turn)  <-------<<
Bolt: 2 ( disadvantage on saves, disadvantage on attacks not vs a skitterer)  <-------<<
Skitterers: 2 (X/X/-5 blind /-5 blind /X/X/? blind/? blind /? blind /?/?/? || X/X/X/X/-15/-15/?/?/?/?/?/? )

Bolt sets to hacking and slashing at the skitterers around her, bringing her axe down twice.

Attack: 21
Damage: 16

Attack 2: 8
Damage 2: 8

Bolt's axe cuts deep into one of them, but it just manages to duck her second swing.

Gramps: 18 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer)
Larry: 12 (disadvantage on saves, disadvantage on attacks not vs a skitterer)
Amorphous Thing: 12 (-43 acided, 3/6 tendrils occupied)
Bryn: 8 
Keller: 8
Robed One: 7 (-115) (invisible, location unknown)
BB: 5 (restrained, disadvantage on saves, disadvantage on attacks not vs a skitterer, enemies have advantage on attacks vs BB until skitterers turn)  <-------<<
Bolt: 2 ( disadvantage on saves, disadvantage on attacks not vs a skitterer) 
Skitterers: 2 (X/X/-5 blind /-5 blind /X/X/? blind/? blind /? blind /?/-14/? || X/X/X/X/-15/-15/?/?/?/?/?/? )

BB thunder waves the tentacle and whatever other monsters he can get in the 15' come.

((DC 12 con save or half): 12 damage 

((BB is able to get two of the skitterers in addition to the tentacle creature.))

7 : Tentacle CON
14 : Skit CON
20 : Skit CON