Not clear on my part. When the party entered the room the area was a clear but early in the round - read into it what you will - new web suddenly appeared in what had previously been a clear space. To leave the original space and for every grid after that characters need to make a DC 12 Strength check. I assumed - obviously incorrectly - you hadn’t powered up the burning sword as it will mean that every character in the room (T, C & Paul) will be imopacted by burning web - i.e. 2d6 damage.

You needed the original dex roll (DC15) for the new web that suddenly enmeshed the characters and the DC 12 Strengh check is to move forward. If you’re balancing on the webs to fight the Spider or Spider Dragon then its a Strenght check - already made {20 rolled} (see post #256) when Corwyn appears and all attacks should be made at Disadvantage.

Hope that is as clear as mud as they say. Any questions reach out!

Corwyn:

(Thanks for the clarification.  The sword has been burning for a while now and at the beginning of the battle I even wrote about him readying his flaming greatsword.)

Dex Save for web at beginning of round:  6

Misty step above the spider dragon and attack at disadvantage as webs burn.

Extra rolls for the first three attacks (originally 18, 27, and 22):  16   11   12 take lower number at disadvantage.

[Hopefully the webs around the spider dragon will burn away and we all drop to the ground.  If not, Corwyn will action surge and attack some more (presumably still at disadvantage)].

(Edit to force through rolls)

Corwyn:

(Note that, even with a miss, the Graze property of the greatsword will still do 5 damage per swing)

With those poor rolls, Corwyn action surges to try and land a hit (still at disadvantage).

Flaming sword with Green Flame Blade:  Attack: 14 Damage: 24

(On a hit do fire damage to webs or adjacent foe:  15

Iterative Attack 2:  Attack: 15 Damage: 18

Iterative Attack 3:  Attack: 20 Damage: 27

At risk of being a right royal pain if Corwyn were to set fire to the web where he is currently balancing fighting the spider he will collapse down a level 

Corwyn takes 7 fire damage as the web around him burns due to his flaming great sword. He then disappears and suddenly appears 20 feet in the air in the web adjacent to the spider. He manages to avoid being caught in the sticky web and swings his flaming sword - which immediately sets the adjacent webs on fire - at the spider. As his weapon hits the spider disappears from sight to appear 30 feet away. As the creature disappears, Corwyn feels himself weakened as if poisoned (DC 15 [Constitution] save required to avoid being poisoned and incapacitated). As the webs he was standing on burn away, Corwyn falls five feet down to the next set of webs (DC 12 [Strength] save required to avoid being restrained.

Two saving throws required:
1) DC 15 [Consitution]  to avoid being poisoned and incapacitated
2) DC 12 [Strength] to avoid being restrained 

I’ll post shortly with Toruun’s attack and then Paul’s action

Corwyn:

Constitution Save:  16

Strength Save:  25

Corwyn:

Scowling as the spider dragon phases away, Corwyn refrains from swinging at empty space and instead looks around for the part of the rod that he saw earlier suspended in the web near the spider dragon’s original location (Passive Perception 16).