A synopsis (& occasional retcon) of the battle so far

(OOC: this is one of the challenges of DDB where I try and make sense of what’s going on!)
 

So here goes…

1. The party was exploring what appears to be some sort of eating hall when Illezenya spotted some sort of creature lurking in the darkness about 60 feet away from him. He promptly illuminated the creature – which turned out to be some sort of huge multi-legged vaguely insect looking creature – with Faerie fire
2. The creature rushed at Illeznya. As it did, Devin lobbed a ball of blue flame Fire bolt at it but due to the creatures jinking movement the ball of blue flame missed
3. A flash of light Guiding bolt emanates from Ebonitus’ outstretched hands and smashes into the creature (18 HP of Radiant damage).
4. The creature lowers its head and bites at Illezenya but at the last minute the half-drow ducks out of the way
5. A huge peal of thunder Shatter rings out from Mullo’s vicinity but while it partially deafens the party and reverberates throughout the chamber the creature simply shakes its head and focuses on trying to bite either Illezenya or Mullo who with a battle cry charges the creature in a rage.
6. D.O.R.K. raises her hand and calls on the power of The Red Knight and a giant sized red long sword Spiritual weapon springs into existence and slashes down at the creature (13 HP of Force damage). Simultaneously she calls in a fire strike /Sacred flame but gets the grid references back to front because it lands nowhere near where the creature is!
7. Worried about the potential impact of being bitten, Illezenya drops his illumination Faerie fire and calls down a series of swirling moons [Tooltip Not Found] to protect him and his colleagues. As the purple illumination fades away, the creature, Illezenya and Mullo are no longer distinguishable to Devin, Ebonitus and D.O.R.K.
8. Devin throws another a ball of blue flame Fire bolt at the creature but this misses. Undaunted he summons a large blue star Spiritual weapon and directs it at the creature (25) - unfortunately due to the darkness surrounding the creature he is unable to target the creature successfully
9. A second flash of light Guiding bolt emanates from Ebonitus’ outstretched hands and targets the creature (8) - unfortunately due to the darkness surrounding the creature he is unable to target the creature successfully
10. The creature pushes through the swirling motes of light (DC 15[Wis] = 7) surrounding Illeznya and Mullo and bites the Shaman picking her up in its jaws and shaking her vigorously. 
11. Mullo valiantly attempts to swing her warpick at the creature but misses. The heat welling up from the creatures gizzards burns the Shaman inflicting a further 11 of heat damage.
12. O.R.K. then swings the spectral sword Spiritual weapon at the creature but due to the fact that it is now in the darkness is unable to target the weapon successfully. (Note what was the other spell you were casting?)
13. Illezenya attempts to touch Mullo’s leg (19) and healing energy flows into her

Ignore the 'CHEAT' Banner up the top I went back and added the extra heat damage to Mullo and that's what set it off. Here is what happened:

• Devin and Ebonitus both missed (due to disadvantage caused by now being required to target the creature in the darkness as a result of Illezenya dropping the Faerie fire
• The creature took only half damage - it made its save - from Illezenya's Spirit guardians
• Mullo took an additional 18 HP of heat damage.
• Illezenya managed to successfully grab hold of Mullo's leg long enough for the healing energy to transfer.

Damage so far:

• The creature (DC 13[Dex] = 20) = 52
• Mullo = 51 and then healed for 18 - balance is 33

Devin you're up

(@Grant: She casted Spirit Guardians as well originally, excluding everyone but Mullo and Ilezenya(since she couldn't see them). But if allowed, change her action to instead don her shield and her free action towards picking up her torch and ideally moving within 15ft of the creature with it, so people can see it again. SInce I very much doubt its gonna move into the light of the giant fire in the room before eating those in the dark with it.)

That’s ok by me

Just making sure I've got it right, because your synopsis of my hit points at the end confuses me. After the creature bit me, my hp is down to 20. If I take another 18, that leaves me with 2. Then I get healed 18, so that takes me back up to 20. Right?

Yes just by touching the creature you take 3d6 of heat damage - unfortunately you got unlucky with additional damage rolled

Devin had cast light on his shield before reentering the cavern (see post #4162) so perhaps not in darkness and not at disadvantage?

Devin moves forward enough so that the creature is 15 feet from him and continues to slam the blue star into the creature

BA - Spiritual Weapon Attack: 21 Damage: 12

He then calls forth Spirit Guardians that also take the form of blue stars. He designates his companions to be unaffected by the spell.

It is only when D.O.R.K. steps forward with her torch and Devin with his illuminated shield that the creature is once again fully illuminated.

I've done some research on multiple instances of Spirit guardians and it would seem from this https://rpg.stackexchange.com/questions/93074/does-the-spirit-guardians-spell-stack-with-multiple-casters and the Players Handbook - "Combining Magical Effects" that only one has any impact on the creature.

Given this  does Devin want to retcon his post.

Seeing that one of his companions already had spiritual guardians attacking the creature Devin changes his mind and casts Bless at 3rd level targeting all of his companions and himself.

"May the Goddess of Magic bless our efforts against this mighty foe!" he calls out as he raises his staff.

He only moves so that he is 20 feet from the creature and the full light of his shield is still upon the creature.

Ebonitus you're up. You're no longer at disadvantage as the creature is no illuminated by the light from Devin's shield and D.O.R.K.'s torch. You're about 30 feet from the creature.

Ebonitus continues to target the creature with [spell]guiding bolt[\spell] .

Attack: 16 Damage: 8

On a hit the next attack has advantage.

Reaction: War God's Blessing to add +10 to Ebon's Attack Roll.

Leery of the shot from Ebon going wide, D.O.R.K makes a sharp gesture and calls upon her goddess to redirect the veering off course bolt.

Ebonitus lines up the creature but at the last minute it jinks to one side, the flash of light seems to sail past the creature but at the last minute it reverses direction and slams back into the creature.

The creature (DC15[Wis] = 17 is largely undeterred by Illezenya's swirling motes of moon and star light and bites down on Mullo's body Attack: 24 Damage: 42 piercing damage (Halved) plus 16 heat damage.

The party can see to their horror the lump as the Shaman is swallowed by the creature.

(OOC: Mullo the heat damage occurs simultaneously with the piercing damage - you will have to make a Death saving throw but remember you have Bless; just because you're unconscious doesn't mean that it no longer works.)

D.O.R.K. you're up

"That is not good." D.O.R.K stated in a very matter-of-fact tone. "It may be time to call in assistance, if at all possible." She adds while directing her celestial armament to slice into the creature's neck as she set down the torch. The moment she rose again, an arm snaps back somehow clutching a javelin of red crackling energy that she almost immediately over launched for the head of the creature as well.

Bonus Action: Attack with (4th level)Celestial Armament.
Attack: 4+18 Damage: 15 of Force.
(Adv. from the Guiding Bolt, if applicable)

Action: Cast (1st level)Javelin of Faith at the Many Legged Creature.
Attack: 4+9 Damage: 11 of Radiant.

Expending Last Channel Divinity for Guided Strike, adding another +10 to her Javelin's attack roll.

Illezenya you're up followed by Devin and then Ebonitus

(The creature will need to make a DC15 Wis save for half of 11 radiant on it's turn.) Ilezenya keeps the star-motes flitting around the creature, and he also calls down a flame-like radiance on the creature, bringing more radiant heat! (DC15 Dex save or take 12 radiant damage from Sacred Flame.)

Concentrating on Bless Devin smashes the blue star into the creature again (BA - Spiritual Weapon)

Attack: 15 Damage: 11 + 4 from Bless

He then casts Magic Missile level 3 for 17 force damage

Both of D.O.R.K.'s strikes hit although right up until the last minute it did look as if his Javelin of Faith was going to miss. 

The creature (DC15 [Dex] = 17 sucessfully dodges out of the way of Illezenya's Sacred Flame

Despite his best efforts Devin's giant blue star swings harmlessly by the creature. Undaunted Devin calls on the powers granted to him by his goddess and five small blue stars slam into the creature.

Ebonitus you're up.

Ebonitus sees the Shaman disappear inside the creature's body and decides this 'encounter is becoming serious. He calls upon the Earthmother to deliver unto him the power to strike this creature down and fires a guiding bolt down upon it with maximum power (Level 3):

Attack: Bless: 4 + 13 Damage: 21

Ebonitus bolt of light smashes into the side of the creature with great power. The creature shakes its head and then (DC15 [Wis] 13) advances on the party's front line of Devin, D.O.R.K. and Illeznya - mouth agape! It selects (1-2 = Devin, 3-4 = D.O.R.K. 5-6 = Illeznya - {6). It selects D.O.R.K. as its target and Attack: 26 Damage: 38 but fails to close its jaws around the metal construct.

Mullo you're up - you take a further 22 of acid damage from the creature's stomach juices

D..O.R.K. over to you after Mullo

Death save #1: 18 + bless: 4

So, I guess that's a save.

With the damage, I lose a death save.

Death save #2: 3 + bless: 2

That's a fail. 1 save, 2 fails.

Wait, would the stomach acid attack be considered a melee attack? If so, I actually lose two fails with that attack. What say you, DM?