Grey Mourner

The Grey Mourner is an NPC found in the east area of the Resting Grounds, and requires Desolate Dive or Descending Dark, as well as the player to find several breakable walls to reach.

Solace Achievement
When spoken to, the Grey Mourner will talk about her dead lover (the daughter of the Traitor Lord), and request the player take a Delicate Flower to her grave, located in the Queen's Gardens This flower will break if the player rides a Stag Station or takes damage.

If the player succeeds in this quest, they will obtain a Mask Shard making her quest required for 100% completion.

Past
The Grey Mourner is probably Ze'mer, one of the five Great Knights, due to the large nail in her home, and comments about serving under the Pale King when shown the King's Brand. Her physical figure also fits the Dung Defender's statues of the other Knights.

Dialogue
Upon meeting:

''Ahhhh.... Me'hon. This world. This cruel, sinful world. Why does che' wake? Why does che' persist?''

Ahhh Le'mer, you could not know of tragedy as complete as che's, true lovers stripped apart, two worlds that could not meet.

''And now meled'lover, dead so long in time. Dead, so far away. Buried, moina? Ai. Amongst its hateful kin that did deny our union, that did reject che's... outside-ness.''

''She doesn't suppose?... Nahlo, Nahlo. Could che' ask? Could che' burden?''

Le'mer, woulds't you, coulds't you deliver gift to her grave?

''The gift is, maybe small? But the trek is long. She rests amongst her kind near our Queen's lush refuge.''

''Would such a thing be done? Che' asks perhaps impossible things? Mad things? For Le'mer to take up such quest would be a kindness che' has near forgotten.''

When shown the King's Brand:

''Le'mer, takes that heavy mantle...Che' wouldn't wish it upon one. Having served that brand at it's great height, che' knows well what suffering it brings. Enough to turn nym'King to flee or fair Knights to mourning.''

Dream Nail:

''Ulll... Nemenoo... Nails and claws... Why must it be so?''

Delicate Flower tactics
Isma's Tear is needed to access the Queen's Gardens. Before accepting the flower, it is recommended to go to the Queen's Gardens and unlock the shortcut to the local Stag Station. This is because the path to the grave of the Traitor's Child is located in the same room as the entrance of said station. If the shortcut is unlocked, the player will have to face only one Mantis Petra in the Gardens on the way to the grave. If not, it's a very long way fraught with difficult platforming and powerful enemies, which are almost guaranteed to destroy the flower.

It is a good idea to do this quest before the Forgotten Crossroads becomes Infected, as the Furious Vengeflies are extremely fast and aggressive, and have a high risk of dealing damage to you. This will limit your offensive options, since you will not be able to get the Monarch Wings without killing the Broken Vessel and triggering the infection. However, on the optimal route, the only threatening enemies you will face will be one single Mantis Petra and a few Spiny Husks. It is recommended to have at least the Great Slash, a Coiled Nail and Fragile Strength for dealing with those.

If you have already triggered the Infection of the Crossroads, it is recommended then to wait to have a Pure Nail and use the Grubberfly's Elegy in combination with Quick Slash and Fragile Strength.

One good route is to leave the Resting Grounds through the Blue Lake and into the Forgotten Crossroads. Once there, head towards the room to the bottom left of the map, the one with the acid pool. Dive into the acid and take the shortcut to Fog Canyon, then head into the Queen's Gardens. Oomas and Uomas are very passive, making the trek fairly easy to do without being damaged as long as you don't collide with anything on your way down through the vertical shaft and refrain from attacking any Oomas. In the Queen's Gardens, take the shortcut towards the Stag Station and you're almost there. There are three spiky sections in the corridor that leads to the grave, which you will need to be very careful with.

You can also transition into Fog Canyon through the Fungal Wastes, as showcased here.