After finding a Dreamer, using the Dream Nail on their body will send the Knight into a Dream Realm that contains a vulnerable form of that dreamer. Killing this form and absorbing it using Focus will break their respective seal.
The Archives also contain a tank full of Charged Lumaflies, presumably there in order to study them. Monomon is hinted to have sourced the Jellyfish found in Fog Canyon, if not only by accident.
Quirrel was Monomon’s apprentice, and she entrusted him with her mask which would serve as additional protection for her once she became a Dreamer. Monomon wants the Seals to break in order to end Hallownest's stasis.
Elegy For Hallownest
The Elegy for Hallownest is a poem written by Monomon the Teacher. Its first four lines are displayed during the opening sequence of the game:
In wilds beyond they speak your name with reverence and regret,
For none could tame our savage souls yet you the challenge met,
Under palest watch, you taught, we changed, base instincts were redeemed,
A world you gave to bug and beast as they had never dreamed.
- From 'Elegy for Hallownest' by Monomon the Teacher
Dream Nail Dialogue
...For diversity, a Seal...
...A world forever unchanging...
...the Seals, must break...
...Better, an end...
Lurien the Watcher
In his Spire, looking over the city.
Lurien's role in Hallownest is unclear, though given that he resided in the Watcher's Spire in the City of Tears with his guards the Watcher Knights and with various servants such as his butler, it is clear he played an important role. At the top of his spire, Lurien has a large telescope that looks down over the City of Tears contributing to his title of "The Watcher".
He became a Dreamer out of loyalty to the Pale King and to protect the City of Tears. Unlike Monomon, he believes that the Seals must remain.
Dream Nail Dialogue
...For King beloved...
...To sleep. To serve...
...Bonds must remain...
Herrah the Beast
In her Den, amidst the deep darkness beyond the kingdom.
Herrah was the former queen of the spider tribe of Deepnest and the mother of Hornet. She agreed to become a Dreamer if the Pale King had a child with her. Since the sire of Deepnest died, it is suggested that Herrah was desperate to have a child of the "honoured caste" due to her being only a common beast; this might have been why she wanted to have a child together with the Pale King. After Hornet was born, Herrah and Hornet were permitted little time together.
Dream Nail Dialogue
...Bound... For brood... For child...
...Fair bargain made...
The Dreamers are encountered as a group in Greenpath and the Resting Grounds. Their physical bodies can be found in different locations, with Monomon's physical body being in the Teacher's Archives in Fog Canyon, Lurien's in Watcher's Spire in the City of Tears, and Herrah's in Beast's Den in Deepnest.
In Team Cherry's previous game, Hungry Knight, there are three masks found on the bosses that look similar to the Dreamers', specifically Monomon's.
In the original concepts for the Dreamers, all three Dreamers looked identical; sharing Monomon's mask and having the body shape of Lurien. The identical masks can still be seen on the archways outside the Temple of the Black Egg, and on one of the Hollow Knight promotional posters. The identical bodies can still be found on the Hollow Knight fountain in the City of Tears.
Cut dialogue exists for all the Dreamers which can still be found in the game files, including a cut version of the Elegy for Hallownest.
After Lurien has been killed all the previously lit candles in his chamber will have been extinguished after exiting his dream.
A test room for the Dreamers exists in the game's files. It contains a figure that looks exactly like the original concept for the Dreamers. It can't be interacted with in any way.
Early in development Lurien was known as "Lurian, The Whisperer". Herrah was called Hegemol, this name was reused for one of the Five Great Knights.
↑Uoma Dream Nail dialogue: "...Monomon...", and Ooma Dream Nail dialogue: "...MONOMON...".
↑Quirrel on the Teacher's Archives: "Despite the sight, I can't help but feel... familiarity?"
↑Quirrel: "To save Hallownest, the Teacher willingly became a seal, but upon herself she enacted an additional protection. Though I cannot recall its happening, I played a part in that feat."
↑Lurien's journal: "For King, for bug, for Hallownest, I head now to my rest."
↑Hornet after Herrah is killed: "We do not choose our mothers, or the circumstance into which we are born."
↑Midwife: "Hers is a tale of tragic exchange. Cost her and her people greatly, though I suspect she bore no regret in making it."
↑ 8.08.1White Lady: "It faced the Gendered Child? She's a fierce foe, strong in mind and body, striking reflection of her mother, though the two were permitted little time together. I never begrudged the Wyrm's dalliance as bargain. In fact, I feel some affection for the creature birthed."
↑Fungal Wasteslore tablet: "This border bounds the twisting, scratching things. Their dead sire, once of honoured caste. Their sealed mother, but the common beast. "