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 colour 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 stagways is longer and 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
 * Many objects and environments have sharper and more contrasting shadows
 * New room transitions have been implemented
 * 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
 * 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 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 grouped in much larger bunches and have more Hp
 * Traitor Lord gained two new attacks, 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
 * 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, can spawn Hivelings on the move instead of stopping, and is faster
 * 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
 * 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 (added for a misconception)
 * If the player encounters the new dialogue from the Dung Defender and then re-enters his home, a dung statue of the Knight can be found next to the others.
 * 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
 * Weaversong's Weaverlings do more damage (up from 2 to 3) with a new attack animation and can stagger bosses
 * Weaversong and Sprintmaster have a new interaction which causes the Weaverlings to move faster, 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
 * New animation for when Grimmchild attacks Millibelle

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 have a new attack sound
 * Bluggsacs have a new disgusting sound when you're nearby
 * Fool Eater have a new attack sound
 * Mosscreeps have a new spawn sound and you should feel ashamed if you hurt them
 * Siblings have a new sound when they spawn
 * Crystallized Husks have a new attack sound
 * Wingmould has a new sound when you hit them
 * Shrumal Warriors have a new death sound
 * Shrumal Ogres have a new death sound
 * Shrumelings have a new adorable sound for when they pop up and run away
 * Volatile Mosskin have new attack and death sound
 * Mosskin have a new attack and death sound
 * Moss Knights have new attack and death sound
 * Gulkas have a new death sound
 * Mistakes have a new death sound
 * Sporgs have a new sound when they pop out to attack
 * Glimbacks have 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
 * Sound for when spikes unsheath in the beginning of the fight with Mantis Lords 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