(Sure, I'll rule the arcana check is a bonus action.  Also, Buppido only moved. He could attack before or after the move if he wants.)

Xilyrah moves (C6) and peers at the webs, attempting to recall any prior experience that would explain the disappearing spider.  However, in the heat of the battle, nothing comes to mind.

Quote from Kerrec >>

(Sure, I'll rule the arcana check is a bonus action.  Also, Buppido only moved. He could attack before or after the move if he wants.)

 (Depends if Buppido used disengage as his action.)

Seeing Buppido attempting to weakly defend himself against additional bites, Xilyrah launches an Eldritch Blast in the direction of the spider to last bite him (C8).

Eldritch Blast: Attack: 13 If successful, force damage: 5

Quote from Quasimodal >>

Quote from Kerrec >>

(Sure, I'll rule the arcana check is a bonus action.  Also, Buppido only moved. He could attack before or after the move if he wants.)

 (Depends if Buppido used disengage as his action.)

 Right. She didn't say disengage so I moved him and no AoO. Moving on... 

Xilyrah obliterates one of the small spiders with a well placed eldritch blast.

Small spider (initiative 16, @B7) will bite at Beegred again...

Bite Attack: 18 Damage: 1 piercing.  If successful roll DC9 CON saving throw or take 3 poison damage.

The medium spider (initiative 13, @E8) continues to bite at Tozer...

Bite Attack: 13 Damage: 4 piercing.  If successful, roll DC 11 CON saving throw or take 7 on a fail, or half as much on a save.

The small spider (initiative 8, @A8) chases after Buppido and bites...

Bite Attack: 14 Damage: 1 piercing.  If successful roll DC9 CON saving throw or take 1 poison damage.

The medium spider (initiative 8, @i10) crawls across the ceiling above Derendil and descends on a strand to bite at him...

Bite Attack: 14 Damage: 3 piercing.

(Beegred's turn)

Buppido, barely managing with all the poison in his system, falls unconscious after the latest bite.

(Beegred needs to roll a CON saving throw)

Beegred CON save vs DC 9 poison: 19 (1 poison if fail, 0 if success)

Beegred will attack, and bonus action attack this round. First he will try the bonus attack on the small spider on him. Attack: 11 Damage: 4

(The streak of bad attack rolls continues...) Let's try the main attack... Attack: 14 Damage: 6

Beegred finally manages to kill the spider harassing him only to have another one (initiative 3, @B8) climb up his leg and bite...

Bite Attack: 9 Damage: 1 piercing.   If successful roll DC9 CON saving throw or take 4 poison damage.

(Narstin's turn.)

Not wanting to swing his warhammer at Beegred's leg, Narstin tries a different tactic and casts Sacred Flame at the tiny spider.

(DC13 dex or 8 damage)

Spider DEX saving throw: 3

The spider on Beegred's leg is scorched to cinders.

The large spider reappears (@A8) next to Buppido.  It grapples Buppido (automatic success) and spins a strand of webbing around his feet, then dropping him to the ground.  It turns to face the back of the cave...

(Derendil, Buppido, Tozer and Xilyrah's turn to act in round 4.  Map and table incoming.)

Note: Spider (Initiative 20) is DEAD.

Tozer becomes aware of the mysterious spider's reappearance, and he considers that particular foe to be the greatest threat to the party's efforts. So, he charges past the medium spider and rushes headlong toward the teleporting arachnid, hoping to smash it with his maul before it vanishes once again...

• Melee attack: 11 | Bludgeoning damage: 11

(I understand that Tozer is now exposed to an opportunity attack by the medium spider)

After missing with his third consecutive swing, Tozer growls in a fury, "Eldeth, we may need your bow!"

Xilyrah is momentarily startled by the reappearance of the giant spider, but gathers herself quickly to fire an Eldritch Blast at it.

Eldritch Blast: 13 If successful, force damage: 5

Buppido death saving throw: 9

The medium spider (initiative 13, @E8) takes advantage of the opportunity to bite at Tozer as he brushes by...

Bite Attack: 13 Damage: 7 piercing.  On a hit, roll DC 11 CON saving throw or suffer 8 poison, or half on a success.

The medium spider misses its opportunity to bite Tozer as the dwarf makes his way toward the giant teleporting spider. 

Seeing the large spider that had hurt him reappear, Derendil charges it and continues slashing at it with his claws.

Attack: 18 Damage: 8

Attack: 14 Damage: 9

The spider (initiative 8, @G10) takes an attack of opportunity on Derendil...

Bite Attack: 18 Damage: 4 piercing

... and manages to sink it's fangs and uselessly inject Derendil with poison.

Derendil rushes the spider (To C9) and scores two painful gashes across the spiders many faceted eyes.

The medium spider (initiative 13, @E8) drops to the ground and crawls up to Tozer (to D8), biting at his legs...

Bite Attack: 6 Damage: 6 piercing.  If successful, roll DC11 CON saving throw.  Suffer 8 poison damage on a fail, or half as much on a success.

... and misses.

The small spider (Initiative 8, @A10) crawls up Derendil's leg and bites...

Bite Attack: 22 Damage: 1 piercing, uselessly injecting poison.

The medium spider (Initiative 8, @G10) drops to the ground and crawls up to Derendil (to D9) and bites...

Bite Attack: 5 Damage: 3 piercing.

 and misses.

(Beegred and Narstin's turns)

Beegred will cast Hunter's Mark on the blinking spider, then take a shot at it with the hand crossbow. After that, move to D6 (if he can avoid opportunity attacks).

Attack: 24 Damage: 10 (includes spell damage)