Online:The Gray Passage

Quick Walkthrough
#Read the book to start the quest.
#Visit the Gray Menhir to start the pilgrimage -  .
#Visit six prayer stones inside the delves.
#Return to the Gray Menhir.

Detailed Walkthrough
Starting the Pilgrimage
On the balcony of the Dragonstar inn Caravan Company Depot there is a book called "The Gray Passage". Read it to gain information about an ancient Nedic hero who completed a pilgrimage called "Gray Passage" and gained the favor of the Stars. You can undertake the same pilgrimage by visiting the Gray Menhir to start it. Optionally, you can first read the old book on the table to gain additional information.

The Gray Menhir is a stone located in the far western side of Upper Craglorn, between Fearfangs Cavern and Sunken Lair. After you have approached it and read the stone tablet,   will start. You will fail the quest if you do not meet the set time.

The Six Prayer Stones
There are six prayer stones you need to visit, all of them in conventional soloable Delves (not Group Delves). The following is just a possible order, possibly one that saves you some time. It starts in Fearfangs Cavern which is very close to the Gray Menhir. From there you will teleport to the eastern Upper Craglorn and visit the three stones there - these are the ones that can take most time. Then you can teleport to the middle of Craglorn and visit the two remaining prayer stones.

The Pledge of Courage inside Fearfangs Cavern - the prayer stone is in a central room, the same where Sepilisk is found. Teleport to the Valley of Scars Wayshrine.

The Pledge of Perfection inside Ilthag's Undertower - clear the first encounter, head upstairs and to the west, towards the rooms of Ilthag. The stone is in the very beginning of the upper part.

The Pledge of Piety inside The Howling Sepulchers - head directly southwest towards the lower parts of the valley. Kill the group of skeletons in front of you and head west up the little ramp, climbing over the waterfall. There is a wispmother you need to fight before heading to the prayer stone. This is by far the quickest one because you do not need to kill so many mobs to get it.

The Pledge of Simplicity inside Exarch's Stronghold - Head over the first bridge and turn west in the first big room to reach the lower part. The stone is in the same area as Agganor. On the northwestern corner. If you want to, you can now use teleport to Skyreach Wayshrine.

The Pledge of Obedience inside Loth'Na Caverns - its in the far northwestern corner. There is a passage to it on the platform where Visskar is.

The Pledge of Vigilance inside Serpent's Nest - in the southwestern corner of the large room with lamia sisters in it.

Teleport to Dragonstar Wayshrine and head back to the Gray Menhir.

Notes
Depending on how much clearing you want to do inside the delves, you should be able to complete the quest in well under an hour.
Two other daily quests can be easily completed concurrently: Iron and Scales and Taken Alive. Both of these quests can be easily obtained after traveling to the Valley of Scars Wayshrine to read the three prayer stones in the easternmost delves, and are easy to complete solo.
A Thieves Trove can spawn behind the Gray Menhir.

Bugs
 

Quest Stages