Polly try to remember if she read what kinds of predators live on this plane and guess what they might sound like. 

What roll would be needed here? Straight intelligence and history? 

As creative as it is, something tells you, devils are not that much scared of monsters. Especially those devils, that can fly. On the other hand, they might get curious - who knows. (Still Tiamat's appearance here is a quite unlikely event - devils would know that)

The monsters here are devils and demons. But, it seems Choir and Polly can try to put to use their knowledge of Religion (DC 15)

Religion check: 20 <— w/o advantage
w/ advantage or separately? 

While walking a little closer to the bridge to plan where the sound should come from, Polly notices something on the bridge. She hurries back to the others and tell them to wait with the diversion for a while. “There are some runes on the bridge over there, and if I’m not mistaken, they are holy symbols of Torm. A cleric of Torm should be able to activate them, causing harm to all fiends on the bridge.” She looks at all of the Elturieliens “I think we will have a real shot at winning against them in a fight if our stealth attempt fails.” 

"I believe they were hunted down first - the clerics of Torm," with a grim expression on her face said Velcora. "Now we know why. They used to pray loudly on each bridge every day. I wish I could remember their prayers."

Choir wonders if she can remember any prayers to Torm...and if Akadi would object to her using them. Religion: 20

With a roll like this she sure does - after all, she can reproduce anything, and she definitely saw Torm's priests performing "bridge prayer" in the past.

Choir wonders how close she has to be to the bridge for Torm to pay attention.  She will cast Fog Cloud near the bridge and quietly try to move into the fog and start chanting the prayer without attracting the attention of the devils. The sounds of battle below should cover the noise of her jingly chain mail, at least...

Polly will cast invisibility on Choir as she walks over. 

"Sounds as good as plan as ever," Adray says to the group before giving Choir a pat to continue.  Turning back to the others he continues, "We need to be ready to support support Choir.  Lulu, can you keep the children safe again?  I'm not as much worried about the Bearded ones as I am about the flying ones.  From what I've seen, magic and silver do the most, avoid fire, poison and acid."  He turns to the dwarfs.  "Do anyone of you have a silvered dagger or blessed weapon?  If I can't borrow it then be ready to come from behind.  I can knock down the bearded ones, maybe the flying ones if they get close, and the rest of you will have to come in to attack."

Fog cloud slowly appeared near the bridge chanting softly in several voices. Devils might not hear the chant over the noises around but fog or smoke intrigued them enough to look, and the missed the moment when runes on the tiles begun to shine with blinding sun light.

There was no battle, there was no need to hide or search for the silvered weapon (that dwarves indeed had) - all six devils with a terrible shriek, heard over the lightnings cracks - were burned to ashes on spots. 

Velcora deeply bowed to adventurers and lovingly caressed the stones as she stepped on the bridge. As much as dwarves were supposed to love combat, she did not look cheated out of her "favourite entertainment".

Lulu played another triumphant melody with her trunk (to much amusement of the kids) and followed.

Choir high-fives Polly, as soon as she isn't invisible any more. ^^

Fran will let out a little grunt of pride for her two teammates who used the power of knowledge to defeat these foes. Brutus would mimic the artificer, holding their heads high as they'd follow behind the others.

Polly bring out her mage hand and high-five Choir back. She breath a light breath that she didn’t have to perform a improvised dragons roar in front of all these people. 

“That was great” she exclaim “but enemies might have seen that light so we better hurry across before any more of them come to investigate. How far is it to your camp?” 

Adray watchdd patiently, knees bent, weight slightly forward, ready to rush to Choir's aid if she needed it.  Then the bridge lit up and all the devils died.  He found himself jogging forward with a slow clap and broad smile.  "Heck of a show Choir.  Heck. Of. A. Show."

He left a moment to enjoy this victory then Polly's words shook him.  "Polly is right, its dangerous to stay here.  Let's head across quickly before more trouble finds us."  Adray waves his our in the universal lets go this way gesture and starts leading the way.

"What camp?" did not understand one of the dwarf Polly's question, but did not insist on explanation and continued his way to the High Halls. Harkina now kept to the rear with kids. She knew the way, of course, like almost anyone here, but only dwarves could show now the safe streets. Dwarves, Harkina and kids were very quiet now, setting example for the newcomers. The only time Harkina opened her mouth was on the bridge, when she asked question, bothering her for some time now: "Who is defending the city?" "Grand Duke Ravengard of Baldur's Gates," answered dwarves, "would not know what we'd do without him."

Either that side of the city was truly in a better shape (devils-invasion wise), dwarves truly knew the best way around, or the devils were pre-occupied with something else right now, but nobody attacked the party and everyone safely reached High Halls, or, as party could now see, what was left of it. 

This grand clifftop alabaster castle was always called the crowning architectural jewel of Elturel. But now, only three of its five watchtowers towers were still standing, and even they appeared abandoned. The wooden gates that once led into the castle grounds have been shattered, leaving a gaping hole in the wall.

The west side of the castle has been reduced to a pile of smashed brick and broken wood. The surviving buildings are blackened by soot. At the center of the castle grounds, the High Hall cathedral stands defiant.

No threats revealed themselves as you enter the castle grounds. But once inside, those on the front spotted two humans in plate armor lying dead on the ground near the shattered gate, blood still oozing from their bite wounds. "Sir Greesan!," gasped Velcora, recognizing the body and began a short pray for the deceased. "But where are the others?" asked another dwarf, looking in surprise around the courtyard, leading to the Cathedral.

Upon seeing the dead guards, Adray's head snapped up to scan for danger.  Whatever killed these men could still be around.  "Group, stay on guard.  The perpetrators could still be near."

Adary will look for threats and then also look for cover.

Perception: 10+1

With Adray's attention is was not that difficult to spot huge dog's paw prints in the ashes around and notice that in addition to the bite marks, there are scorch marks as well on the bodies.

Choir checks to make sure the fallen knight is actually dead.  If not, she will try to heal him.  (If he is dead, she casts Gentle Repose )

Both men are dead, but considering what the group saw until now, the precaution might be useful.