Kaelen perception: 24

Asariel perception, 14

Arista perception 16

[[Mathias Disadvantaged Perception: 16]]

There are two large vaguely avian shapes circling around the cave. [[DC 15]]

[Brekka: 7]

Arista points skyward at the circling shapes, murmurs a guttural rune word, and draws the lingering drizzle into a crackling shell of ice that plates her cuirass (Armor of Agathys) while a thin film of frost sinks into her skin to blunt incoming talons (Resistance against Piercing). Shield up, she strides out into the open, stamping the mud and barking a challenge to fix the perytons’ gaze on her, hoping to give the others clear shots. Her free hand, ready to grapple one of them should they come close, or react with a defensive spell.

preparation before stepping outside: casts Armor of Agathys and Resistance against Piercing
move outside
action: ready grapple with an unarmed strike at the first enemy entering her range (DC 12). 

Asariel sees nothing, but then once Arista points them out he can just about make out something but the weather is making it hard. He waits in the cave mouth for the creatures to get closer before acting. (action, dodge) 

Kaelen moves out of the cave into the darkness, readying an arrow to loose at the first Peryton to attack Arista.

Ready action: longbow to hit 9 -.- (Oh, forgot about his Invisible advantage. Does the storm cancel that out?)

Kaelen: Invisible advantage? Are you talking about the invisibility to creatures depending on darkvision? That never applied, the lighting was dim light at it's worst, and is now functionally normal illumination.

Arista: I'll let you cast the spells and move outside before initiative, but you would not be able to ready that action, as moving outside triggered initiative, and bonus, cantrip as action and move is a full round.

However, the perytons are 400 feet away, so you have a little time. 

Once Arista steps outside, the perytons screech and begin diving, wickedly pointed antlers first.

Torban dodges, effectively skipping his turn.

Mathias is up, followed by the perytons. Anyone who already declared an action will need to at least post that they are not changing their action after the perytons go.

Mathias empowers his staff with primal power and move to edge of the cave, waiting for the enemy to be within the range of a starry wisp.

[[Bonus action - Shillelagh. Action Ready to fire off Starry Wisp when a Peryton is within 60 ft.  AC 19 for 5 Radiant damage if the action triggers.]]

((Oh, sorry. Around what time is it?))

[[It's like 2-3 in the afternoon. You've gotten about half as far as you planned so far.]]

The perytons, seeing an exposed humanoid, continue their dive, they are now about 280 ft. away, much of that is vertical.

Everyone but Mathias is up.

Handy Planning Guide:

Round 1: Perytons end 280 feet away.
Round 2: Perytons end 160 feet away.
Round 3: Perytons would end 40 feet away unless tactical conditions change.

None of Asariel's options work for greater than 60ft apart from his crossbow so he will fire at one of the Perytons (in disadvantage range). 23 to hit for 7 piercing damage. 

"Here we go!" Brekka roars as he steps outside and prepares for the oncoming creatures.

"Strike fast and true, friends. To a glorious battle!"

A bright, white glow emanates from the emblem on his shield before it splits into three motes of energy that streak out and settle on the shoulders of Arista, Kaelan and himself.

Action: Bless

(Sorry, will be traveling today and just write the actions without any prose.)

Round 1: as above (and can't do anything else)
Round 2: readies sorcerous burst for when the first enters 120ft range, hits 18 for 8 cold damage
Round 3: readies a grapple (unarmed strike) for the first peryton that attacks, DC 12

Armor of Agathys (5tHP), Resistance (3/10 rounds, -1d4 piercing damage)

She will take the readied action, unless she receives a brutally high hit (like 15+ damage) with the second enemy still coming at her, in which case she would use shield if it helps against the attack.

Kaelen will try loosing an arrow at one of the flying creatures.

Action: longbow to hit with disadvantage 13+4 bless= 17, damage 11. If that hits he'll add a dreadful strike for 11 psychic damage.

Asariel and Kaelen leave one of the perytons extremely hurt as both attacks hit the same wing.

That peryton veers off to the northeast, flying away from the party. The other continues its dive, ending about 160 ft. away.

Torban moves next to Arista and readies an action to strike if a peryton comes within 10 feet.

You all are up.

Kaelen frowns as the wounded Peryton flees; so they are intelligent, not simply bloodthirsty. That will make this more difficult. He turns his attention to the one remaining and moves 10 ft closer to get a better shot.

Action: longbow to hit 11+1 bless=12, damage 9

Asariel also fires at the other one, still a bit challenging due to the range but Asariel focuses on the dance of the battle, using the flapping wings as a beat to flow with, timing the release of the bolt (19 to hit for 9 damage).