Lifeblood

Lifeblood is an update to Hollow Knight which was officially released on the 20th April, 2018, after a period in public beta. As it was not one of the three originally planned DLC updates, it did not release with a teaser. The update focused mainly on enhancing technical aspects of the game, bringing upgrades and optimizations of various kinds. (warning: SPOILERS ahead)

Bosses

 * Hive Knight

Mapping Tools

 * Map Markers

Main menu

 * Players became able to change the background of the title screen (Extras section)
 * The menu styles include:
 * 1) Classic (Unlocked from start)
 * 2) Hidden Dreams (Hidden Dreams update)
 * 3) Grimm Troupe (The Grimm Troupe update)
 * 4) Lifeblood (Lifeblood update)
 * 5) Infected (Unlocked by beating The Hollow Knight)
 * 6) Void (Unlocked by beating The Radiance)
 * 7) Steel Soul (Unlocked by completing Steel Soul Mode, any ending)
 * A changelog was added, which includes information about every major version of the game (Extras section)
 * Blur quality can be changed through the Video menu

Visual Changes

 * The Hive area gained a complete visual redesign
 * Improved color pallet for many areas
 * The Zote Statue implemented in the Hidden Dreams update now only has four candles, but will still turn gold after ten defeats
 * Lighting is more bright and livid, with a more blueish hue
 * Small foreground spiders may appear and scuttle away from the player's feet when exploring Deepnest
 * Animation for traveling the stagway has been tweaked with hanging lights, and is now able to be skipped
 * Mid ground and background perspective movement is a lot smoother and clearer
 * Using Crystal Heart Dash now gives off a flash of purple light (the animation may also be quicker)
 * Tram pass slots now have a glowing yellow light
 * Calling and riding the tram now has a rippling effect overlay
 * Sections of Ancient Basin and Kingdom's Edge are noticeably more blue toned
 * Shadows for many objects, unkillable organisms (spike worms on the floor) and environments have sharper and more contrasting shadows
 * New transitions have been implemented and the scale and zoom of the camera may have been tweaked (?)
 * The lights of the houses in Dirtmouth are more vivid and noticeable
 * Forgotten Crossroads loses saturation and is more gray scale once it turns into the Infected Crossroads
 * The Knight now has a new burst animation when taking damage
 * Damage taken from coming in contact with spikes triggers a unique burst animation
 * Damage taken that deals multiple masks now triggers a new burst animation
 * This animation can be seen when The Knight is killed
 * Dying in Steel Soul Mode has a unique animation
 * The lighting fading and transitioning from area to area has been modified
 * Entrance to the False Knight boss fight now has a furnished gateway
 * Decorative mushrooms in the foreground of Fungal Wastes now react to the player walking over them and let out a puff of spores
 * Smacking and breaking many decorative pieces will now cause a ripple effect overlay (?)
 * The greenhouse where the two Shrumal Ogres are fought is now a vibrant green
 * The arena for the Lost Kin loses saturation and is more gray scale
 * There are now gifs for the loading screen
 * Areas in the City of Tears now use a purple hue and tint
 * New icon for getting an achievement

Enemy Changes

 * Hivelings are now much larger and have more Hp
 * Traitor Lord gained two new attacks and now deals two masks of damage and is a lot faster
 * The Hollow Knight's diving flame pillar attack now does two masks of damage
 * The Collector can now be staggered
 * Nightmare King Grimm is now only accessible while wearing the Grimmchild charm
 * Soul Master's size has been decreased considerably (possible glitch)
 * Many enemies and bosses have had tweaks to their colours making them more vibrant against the environment
 * Soul Tyrant can no longer be staggered
 * Hive Guardians do two masks of damage
 * Volatile Mosskin Explode faster upon death
 * Desolate Diving onto Flukemarm no longer functions
 * Husk Hive is larger and can spawn Hivelings on the move instead of stopping and are faster
 * The Hive now contains a higher amount of enemies
 * Aspid Hunters are now a more brighter orange to match the rest of the infected enemies
 * False Knight's stagger animation is faster
 * Fungified Husk's spore cloud is significantly bigger
 * Traitor Lord has a visual cloud of infection surrounding him
 * Traitor Lord has a new death animation when fighting with Cloth
 * The bodies of the Watcher Knights have been rearranged
 * Belflys will leave their skull behind after death, if they are on screen when they suicide it will still count towards the Hunter's Journal

NPC Changes

 * The Dung Defender now has extra dialogue after the player beats the White Defender five times
 * Divine now has extra Dialogue when the player talks to her while wearing the Defender's Crest
 * If the player encounters the new dialogue from the Dung Defender, leaves his home and returns there is now a seventh dung statue in addition to the five knights and the king; one of the player hollow knight
 * Dreamer sequence after first Hornet fight has new animations
 * New Dialogue from Hive Queen Vespa once the Hive Knight has been defeated
 * Bretta's previously added dialogue now coincides with the amount of candles lit on the zote statue, she will also run away after four defeats of Grey Prince Zote

Charm Changes

 * Flukenest's damage has been nerfed and now costs 3 notches
 * Weaverlings do more damage with a new attack animation and can stun bosses
 * Weaversong and Sprintmaster have a new interaction which causes the weaverlings to move faster and wander a bit farther from the player and jump higher
 * Weaversong and Grubsong now have a new interaction which causes damaged weaverlings to give the player soul
 * Fury of the Fallen's activated effects has a new design and instead of a red circle highlighting the player, the player now emits a red smoke

Audio Changes
(note: taken from the /r/HollowKnight subreddit, editing is needed, credit due here: https://www.reddit.com/r/HollowKnight/comments/86tqh5/spoilers_compilation_of_changes_with_lifeblood/)
 * Fungoons has a new attack sound.
 * Bluggsacs has a new disgusting sound when you're nearby.
 * Fool Eater has a new attack sound.
 * Mosscreeps has a new sound and you should feel ashamed if you hurt them.
 * Siblings has a new sound when they spawn:
 * Crystallized Husks has a new attack sound:
 * Wingmould has a new sound when you hit them:
 * Shrumal Warrior has a new death sound.
 * Shrumal Ogre has a new death sound.
 * Shrumelings has a new adorable sound for when they pop up and run away
 * Volatile Mosskin has new attack and death sound.
 * Mosskin has a new attack and death sound.
 * Moss Knights has new attack and death sound.
 * Gulkas has a new death sound.
 * Mistakes has a new death sound.
 * Sporgs has a new sound when they pop out to attack
 * Glimbacks has a new and louder scuttling sound whilst patrolling
 * New audio for locked arena/waved-styled fights
 * New sound for when you place a Dream gate in an invalid location
 * Dreamshield makes a 'tink' noise when it passes over a piece of geo
 * Dung Defender and White Defender now audibly pounds his chest when he transitions
 * Mantis Lords sound for when spike pops up in the beginning has been changed.
 * New OST for arena/wave-styled fights in City of Tears
 * New sound for breaking mushroom flora in Fungal Wastes.
 * New sound for breakable pipes and such in Royal Waterways.
 * Bretta is now audibly scared when you first find her.

Misc. Changes

 * Inventory dropping (speedrun strategy) has now been removed
 * Controls for The Knight seems to have been 'tightened' and the knight also seems to fall faster (need confirmation)