Lurien is at the top of the Watcher's Spire in the City of Tears, behind the Watcher Knights. You need the Elegant Key from Sly, which costs 800 Geo, to get into the Spire at all. Monomon is in the Teacher's Archives on the eastern edge of Fog Canyon, behind Uumuu, the jellyfish boss you cannot damage with your nail. Herrah is in the Beast's Den, deep in Deepnest, past a long stretch of collapsing floor and the Distant Village.
You need the Dream Nail before any of them count. Killing the guardian boss is only half the job; you then Dream Nail the sleeping Dreamer to break the seal.
How to collect them
Do Monomon first if you want the easiest route. Fog Canyon is close to the Fungal Wastes and Uumuu is more of a puzzle than a damage test, since you have to bait Quirrel into striking the core. Lurien second, once you can afford the Elegant Key. Herrah last, because Deepnest needs the Lumafly Lantern and the walk in is long.
After the first Dreamer falls, the Forgotten Crossroads becomes the Infected Crossroads with stronger enemies, so clear the grubs, the Mask Shard and the Brooding Mawlek there before you commit. Filter this map to the Dreamers category to see the three seals and their guardians together.
What you get
The three Dreamers are worth 3 percent of the 112, one each. Breaking all three seals opens the Black Egg Temple and lets you fight the Hollow Knight for the standard ending. With the Void Heart equipped, the same door leads to the true ending and the Radiance instead, which is why most players hold off until they have the Kingsoul from the White Palace and the Queen's Gardens.
All 6 locations by area
City of Tears (1)
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Top of the Watcher's Spire, City of Tears
The first Dreamer most players reach. Beat the Watcher Knights, ride the lift to the top of the Spire, and Dream Nail Lurien to break one of the three seals on the Black Egg.
Resting Grounds (3)
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Dream altar above the Resting Grounds bench
The key item that lets you read minds, collect Essence and enter dream worlds. Walk up to the glowing altar in the Resting Grounds and accept it from the Moth Tribe, then talk to the Seer.
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The Seer's chamber, Resting Grounds
Places a warp point anywhere and lets you return to it from any bench, which saves enormous amounts of backtracking. The Seer teaches it once you bring her 900 Essence. It does not add completion percentage.
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Seer's chamber, Resting Grounds
The upgraded Dream Nail that can pierce the Hollow Knight and Grimm, which is required for the true ending and Nightmare King Grimm. The Seer awakens it once you bring her 1800 Essence.
Fog Canyon (1)
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Teacher's Archives, Fog Canyon
Sealed inside a tank at the top of the Archives. Defeat Uumuu with Quirrel's help, then Dream Nail Monomon and Quirrel will unlock the tank for you.
Deepnest (1)
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Beast's Den, Distant Village, Deepnest
Sleeps at the end of the Beast's Den, which is far easier with the Lumafly Lantern. Fight through the web maze and the ambush room, then Dream Nail her to break a seal.
