(Used the Game Log roller... The dice gods still toy with us...)

WIS save vs. DC 13: 14  The creature manages to resist the desires and impulses pushed at its brain.

 "Cowardly frogstrosity!" Jorin shouts as the creature flees via sorcerous means.

Feldspar grimaces at the bite, feeling herself get closer to death. She closes her eyes, pushing away the disgust of her current surroundings, and feeling alone. She calls out mentally. "Shadow. I don't want it to end like this. Not like th..." and with a furious swipe of her arms she casts sword burst.

9 slashing damage to everyone within 5 feet of her -- per the map, and the size of the toad, and her placement INSIDE, would that mean the toad takes more damage than that? Three squares' worth perhaps?

DEX 15 save, though would there be an argument that it's rather hard to make the save given her placement inside?

(It is still only 1 creature, even though it takes up 4 square on the map.

Cantrip doesn't require seeing a target, so that works.
I had the same thought about the DEX save... The ruling is delayed, while I search around and see if I can find how other people handled this kind of situation.)

(EDIT: For this particular situation and this specific cantrip, the frog-osity will take auto-damage.

I may rule differently for other situations or different spells.
There was an interesting Reddit discussion about purple worms, where one person that points out a DEX save is not necessarily dodging out of the way.
"A creature's hit points and dice results are not necessarily because of conscious effort. It is also an indicator of luck. Rather than the creature actually dodging the Disintegrate, its muscles around you might contract and twist your arm, causing your spell to fizzle. Or sudden movement, darkness and general disorientation might just mean you hit a spot covered in stomach acids, causing less damage from Fireball.")

Ok, ok, thanks for the auto-fail. Still think there'd be more damage, but far be it from me to haggle!

B1's guts are writhing, like it has indigestion from what it ate (9 force damage). But it keeps Feldspar in its guts.
Feldspar takes 10 necrotic damage at the start of its turn.

It also seems to understand the Common language, because it makes a rude hand gesture to Jorin after his outburst.

B1 will make a bite attack at Drusk (AC 18).
If it hits, it will then move north of the nearby pillar (to the squares A,B 8,9), dragging the grappled/restrained Drusk to C7.
If it misses, it will take an AoO from Drusk, moving north around that pillar and ending up at C,D 11,12.

• Bite Attack vs Drusk: 14 Damage: 19

Jorin, Quillary, and Drusk are up next.

Quillary growls as her monster disapparates, and turns to discover it eating Hildigrim. She points at it, and her spores shoot out, grasping at the thing, then she moves in to engage one of the monstrosities.

Action: cast Chill Touch at B2. 25 to hit, for 4 necrotic damage.
Move: If B2 is not dead, she moves as close to melee range with it as she can. If it is dead, she instead closes with B1 as much as she can.
Reaction: if she is within 10' of a monstrosity (see above which one) when it starts its turn, it must Con Save vs DC 16 or take 4 necrotic damage from her Halo of Spores.
Bonus Action: nope

Jorin charges after B2 and lays into it with his blades.  He has enough movement (40 ft) to get over to B1 as well if B2 dies at any point before/during his turn.

• Movement: 15 ft to B2
• Action: Battleaxe!
  • First Attack: 14 Damage: 14
  • Second Attack: 25 Damage: 10
• Bonus Action: Moonblade!
  • Attack: 23 Damage: 13

Quote from Guiltysparc >>

He has enough movement (40 ft) to get over to B1 as well if B2 dies at any point before/during his turn.

(B1 moved over to column 11.)
Jorin moves west to attack B2 that was going after Hildigrim.
The first swing was brushed aside, but the second one hits solidly.
The creature was already severely wounded, and this axe swing was enough to cause it to croak. (Please don't hurt the DM, but I had to say that.)
Jorin then uses his remaining 25' of movement to reach E8, to give chase to B1.

Quillary was targeting B2 to help out Hildigrim, but with its death, she switches to B1 to the east.
She moves 5' to get within spore range.
The Chill Touch gives it a friendly pat on the head for 4 necrotic damage.
B1 CON save vs. DC 16 spores: 16

After using his AoO, Drusk moves east and around the side of the pillar, to attack the creature again. (Feldspar is well protected from external attacks.)
(Attack rolls in Game Log. Divine Strike is the extra d8 radiant damage.)
Drusk's AoO glances off harmlessly (14 to-hit), but after he catches up, the next swing with Joybringer II is a solid blow (24 to-hit, 10 bludgeoning + 6 radiant).

Start of a new round.

B2 is dead, releasing Hildigrim from its jaws.
B1 has taken a cumulative 34 damage so far. Enough to show wounds, but this one is even beefier than its ugly expired buddy.
Feldspar is still blinded and restrained in its gut.

Hildigrim and Feldspar are up. (Then B1 goes.)

Feldspar's resistant to necrotic damage so she takes 5 instead of 10.

Can Feldspar try to break free? Athletics check? She's not very athletic, but don't think ability checks are affected by her conditions. Attacking from the inside's at disadvantage, so if she can't try to break free, I guess attacking at disadvantage will be what she does, since sword burst didn't work very well.

Ack, Jorin again yells at the other, now also fleeing, frog-strosity about it being a coward!  lol

Quote from rkmobius >>

Feldspar's resistant to necrotic damage so she takes 5 instead of 10.

Can Feldspar try to break free? Athletics check? She's not very athletic, but don't think ability checks are affected by her conditions. Attacking from the inside's at disadvantage, so if she can't try to break free, I guess attacking at disadvantage will be what she does, since sword burst didn't work very well.

(Yes, the damage should be reduced to 5. Thanks for pointing that out.

The creature description doesn't have a mechanism for someone to break free from swallowing.
That was part of the reason why I was okay with Sword Burst doing damage without a save.
I will say that Feldspar cannot die from this. The necrotic damage turns off when the swallowed creature is unconscious (and stable).

That is the trade-off, if you think attacks-with-disadvantage will do more damage than an auto 2d6 every turn.)

Hildigrim squirms out of the dead creature's limp grasp and grunts as he moves around to see the other froggy still hopping (F5). He draws an arrow and fires after asking Volo to lend him aid once again. Then, seeing Jorin about to reach the monster, the halfling offers some verbal aid in the barbarian's next attack.

Attack: 23 Damage: 22

Feldspar tries to reach up for the opening to the mouth and apparently can't find purchase, so she again casts sword burst for 10 damage again. 

(BTW, I am assuming that Shadow Step only works on the creature itself.
If it still worked when someone is swallowed, then this thing would be shadow-step + movement + dash, which seems wrong to dine-and-dash.)

B1 does a tongue lashing at Quillary. If it hits, does not attempt to move her.

• Attack: 22 Damage: 12 necrotic

Makes a bite attack at Drusk.

• Attack: 15 Damage: 15

(Another post shortly, with an updated map.)

B1 was hit by Hildigrim's arrow and force-stabbed by Feldspar's cantrip for 33 damage together.
Quillary was able to slap aside the tongue with her shield.
Feldspar takes another 5 necrotic damage (reduced from 10) at the start of B1's turn.
The creature has Drusk in its jaws (15 damage). It can't swallow him (Feldspar is already inside), but he is grappled and restrained by the teeth.

B1 moved to the north slightly (dragging Drusk with it) to get more cover from the nearby pillar.

Jorin, Quillary, and Drusk are up.

Jorin contines to run after the creature, vaulting over Drusk and cutting the frogstrosity off before hacking away at it some more!

• Movement: 25 ft to B12
• Action: Battleaxe
  • Attack (w/ adv from Hildi): 24 Damage: 14
  • Attack: 28 Damage: Unable to parse dice roll.
• Bonus Action: Moonblade
  • Attack: 27 Damage: 12

Booo on the damage rolls

Quillary grumbles and steps to the side, pointing her spores at the monster.

Move: 5' east to see the monster
Action: Cast Chill Touch at it. 11 to hit, for 4 necrotic damage. Lame.