Aldrich stays hunched down on the right side of the door, just next to the wall

(I believe rapier-man is blinded due us being in complete darkness - PHB. But he is out of range of of Muirgurn, who stays on the opposite side of the door to Aldrich. About the most useful thing Muirgurn can offer is guidance, in case anyone is doing anything that requires an ability check.)

(Aldrich could brobably use it for a possible athletics(grappling) check)

First, to answer Peter's questions:

1. Yes, Peter (and everyone else in the room) is in an area of darkness and would have advantage on attacks against a target that can't see him.

2. The darkness extends to halfway down the hall. (On the map, grey is darkness and the brownish orange is dim light.) Anywhere within the area of darkness, you are unseen by creatures that don't have darkvision or some way to see you. Peter could move 25 feet forward (or farther) to get within range of the man and still be unseen within the area of darkness.

So as far as I understand, Aldrich is to the right of the door readying the action described in post #153, i.e. to grapple anything that comes through the door.

Is Muirgurn keeping sacred flame ready with the trigger described in post #144, or does he cast guidance (on Aldrich) or do something else?

Muigurn offers the benefit of Moradin's guidance to Aldrich.

Aldrich, you have a guidance die for the next minute as long as Muirgurn maintains concentration.

Peter and Razil, the man with the rapier is moving forward, intent on attacking whoever is in the room. Feel free to post what you do on your turn.

Razil will back up next to Peter, in front of the hallway, and cast Eldritch Blast down the hall into the man with the rapier.

Attack: 12 Damage: 9 

After letting loose his spell, Razil will use the rest of his movement to back down the hallway another 10'

Peter will move forward to get within 80 feet of the man with the rapier and lets loose a shot from his bow. (( 21 to hit doing 7 damage plus 6 sneak attack damage, with the crit I'm pretty sure that's 21 damage)) then He'll dip off to the right of the doorway, out of line of sight of the hallway

Critical hits double the damage dice, so roll another 2d6 for the extra damage.

Peter's arrow hits the man squarely, penetrating his mail and momentarily knocking the wind out of him, but the resolve doesn't fade from his face. Razil's beam of energy is deflected by the man's habergeon, and he charges forward, grabbing a torch from a sconce on the wall. "Shooting from the dark like the cowards you are! I'll soon lay bare your strategies," he shouts as he dashes up the hallway towards you, catching Peter in dim torchlight that comes in through the doorway.

Everybody's up again!

Edit: Oh, and Muirgurn, you can ignore that door for movement purposes. I just didn't want to redraw it.

((extra damage 4 )) peter gives a slight smile and a slight bow, then turns and dashes back into the shadows near Razil. He then nocks and pulls another arrow, but holds it, waiting for his allies to strike, in hopes of getting another well placed shot ((I'm going to hold my attack until It qualifies for another sneak attack))

Now Muirgurn steps out and sears him with sacred flame, before stepping back into the shadows behind the door.

(DC13 Dex or 5 radiant damage)

Razil mutters under his breath, quietly cursing the being that granted him his powers.  Focusing, he fires off another Eldritch Blast at the oncoming foe.

Attack: 25 Damage: 6 

The man attempts to duck under the radiance that descends upon him as he charged towards the doorway. Dexterity save: 5

Is Aldrich still readying to grapple?

Peter, you are unseen as soon as your movement takes you out of the radius of dim light giving you advantage and sneak attack damage on your attack, so your "readied" attack can happen on your turn.

The man's skin burns with radiant energy and the arrow wound in his side begins to bleed profusely, causing his mail to glisten with blood.

Aldrich charges the man at full speed and tries to grapple.
Athletics: 15 (with guidance)

((oh sorry, I wasn't sure if it still counted, Peter will loose his attack from the shadows then. 15 to hit doin 13 damage))

Finding himself suddenly seized, the man tries to counter Aldrich's grapple. Strength (Athletics): 22

Your opponent nearly evades your grasp, Aldrich, but Moradin gives you the upper hand, and he is firmly grappled by you just before Peter's arrow flies out of the darkness, piercing the man's throat and sending a torrent of blood down his habergeon and his ruby belt as his lifeless hand drops his rapier, shining brightly in the light of his torch before it too falls into a pool of blood. Then, everything is quiet. 

without missing a beat Peter nocks another arrow, and turns his aim down the hallway once more, "there's another one down there, doubt there's much sense trying to talk to that one. who wants to lead the way?" as he says this he steps forward and rolls the torch around in the newly formed pool of blood, making sure to snuff it out.