Broken Vessel

Attacks
In addition to having a similar appearance to The Knight, the Broken Vessel also shares similar moves:
 * Slash: Dashes forward and swings its nail.
 * Air Slash: Jumps slightly off the ground before performing a Slash attack
 * Leap: Jumps in a wide arc to move quickly throughout the arena, dealing contact damage
 * Nail Flail: Rapidly swing their nail left and right.
 * Infected Slam: leaps into the air before stopping and dropping down to the ground, dealing contact damage and spawning 4 orange blobs to fly upwards out of the ground
 * Infected Flurry: Flails in place, causing a pattern of orange blobs to fly out from the ground over the whole field.
 * Spawn: An Infected Balloon will appear from time to time and float toward the player, dealing contact damage.

Tactics
The Broken Vessel's attacks can be hard to dodge. Quick Focus is highly recommended for this battle. Mark of Pride or Longnail can help and Spore Shroom is perfect to get rid of the orange blobs while healing as well as dealing extra damage to the boss.

Using the Descending Dark Spell during its most powerful attack (when they shake their head and the blob comes out) will deal a lot of damage to it and prevent the player from taking damage at the same moment. When they're stunned, if the player doesn't need to heal, charge Great Slash (if available) and start the following phase either with that or the Descending Dark spell. A good opportunity to deal a lot of damage is to strike them repeatedly during Infected Flurry.

Joni's Blessing is not recommended, unless going for a face-tank strategy.

Upgrading the Nail and gathering more Mask Shards can be helpful if stuck on this boss.

Lost Kin
After acquiring the Dream Nail, the player can re-enter the arena and strike the Broken Vessel's corpse with the Dream Nail to be taken to an arena to fight the Lost Kin.

Attacks
Like the Failed Champion and Soul Tyrant, the Lost Kin has the same moveset as the Broken Vessel, but will spam their attacks near constantly. The Infected Balloons will also spawn more frequently, and Infected Slam spawns six blobs rather than three.

Defeating the Lost Kin grants 400 Essence.

Tactics
Mostly the same as the Broken Vessel, only face-tank strategies won't work.

It's recommended to get rid of the Infected Balloons quickly as they can hit the player when distracted or attempting to Focus. The Balloons are also a good source of SOUL. The Defender's Crest can also be useful since it will kill any approaching balloon instantly. Spore Shroom can achieve a similar effect.

Stay close to them, but never too close. When they perform Infected Slam, jump and move to the left/right slightly just enough to dodge its fall, so the player won't take damage while ending in a convenient position to attack.

The Lost Kin often travels to points in the room by jumping, because of this it is advantageous to mainly remain on the floor and travel under him while the Lost Kin jumps over you, the main reason to jump is to dodge the Slash attack.

As with the other two Dream Bosses, Fragile Charms will not break upon death, meaning the player can take advantage of Fragile Heart and Fragile Strength without any risk.

Trivia
Разбитый Сосуд
 * The Broken Vessel bears strong resemblance to The Knight, and has the same death screech as the Hollow Knight. This implies the Broken Vessel is indeed a Vessel created from Void like them.
 * Its attacks resemble moves performed by the Knight, such as Dash Slash and Desolate Dive.
 * Upon defeat, The Broken Vessel will reach out to the player before dying, and after defeating Lost Kin, their ghost will wordlessly bow to the player before being absorbed.