What should I buy?
Stay in the black Market for 1 minute


Brain at Full Throttle
Spend 5 minutes in the Black Market


Too much of a Talker
Keep talking with the Headless

Just talk to the Headless shopkeeper until the achievement unlocks

Inventory Management
Re-stock continuously

Refresh the Items shop, I got the achievement after 5 refreshes

Castle Restoration Plans
Restore the Demon Kings Castle to its original form

Rescue the Death Knight on the 4th level of the game, return to the Demon Kings Castle and pay him to restore the Castle. Fully restoring the castle will cost a lot though..

The Dwarven Chandelier
Reach the top of the chandelier in the Demon King Castle

But first you need to buy the chandelier from the rescued Death Knight

Rules of Etiquette
Reach the top of the throne in the Demon King Castle


Go Home...
Rescue the Demon King and return to his Castle

Beat the game and then click continue

Cold Blooded
Defeat the First Hero without rescuing any NPCs


The Hero Killer
Kill 100 Heroes


The Skeleton King
Defeat the First Hero with the Little Bone

Defeat the First Hero with Skul's original head. You can have a second skull, but switch back to Little Bone for the final hit ;)

Skul Focus
Die from a trap


Skul Rookies Welcome
Play on Rookie Mode

You don't actually have to play on rookie mode.
Just go the settings and turn easy mode on, get the achievement and if you want, turn it back off.

The Legend Begins
Return after experiencing death

Return to the Demon King's Castle after dying

Legendary skull achievements

A Stylish Entrance
Acquire Super Rock Star

Rock Star > Super Rock Star

All Soul's Day
Acquire the Savior

Grave Digger > Undertaker > Soul Bearer > Saviour

An Endless Appetite for Research
Acquire Dark Alchemist

Alchemist > Grand Alchemist > Dark Alchemist

An Honorable Name
Acquire Royal Guard Captain

Skeleton-Sword > Guard > Royal Guard > Royal Guard Captain

Arms Expert
Acquire Eternal Mummy

Mummy > Revived Mummy > Revived Ancient Mummy

Beheaded
Acquire the King

Prisoner > The King (Dead Cells)

Burning Soul
Acquire Soul Knight

Living Armor > Soul Knight

Charge! Again! And Again!
Acquire Vanguard Captain

Skeleton-Pike > Foot Soldier > Pikeman > Pikeman Captain

Demon King Castle Gargoyle
Acquire Golden Gargoyle

Gargoyle > Bronze Gargoyle > Golden Gargoyle

Destructive Instincts
Acquire Destroyer

Predator > Destroyer

Do you fear death?
Acquire Davy Jones

Already Legendary

Dominating Size
Acquire Ghoooooul

Ghoul > Ghoooul > Ghoooooul

Explosions are a Work of Art
Acquire Demolitions Expert

Skeleton-Bomber > Skeleton Bomber > Demolitions Expert

From the Deep Abyss
Acquire Abyssal Monarch

Great Warlock > Abyssal Monarch

Ghostly Elusiveness
Acquire the Overlord

Stone Monkey > Monkey King > Immortal King > Overlord

Guardian of Nature
Acquire Elder Ent Skull

Ent Skull > Old Ent Skull > Huge Ent Skull > Elder Ent Skull

Praise Me!
Acquire Spirit

Gene > Apprentice Gene > Magnifico

Rider Instincts
Acquire Nightmare

Rider > Phantom Rider > Nightmare

Sealed Power
Acquire Arch Lich

Already Legendary

Start the Show
Acquire Joker

Clown > Pierrot > Joker

The Back Alley King
Acquire Desperado

Petty Thief > Thief > Wanderer > Outlaw

The Best Offense is Defense
Acquire Guard Captain

Skeleton-Shield > Shield Bearer > Guardian > Guardian Commander

The Dark-Armored Knight
Acquire Dark Avenger

Dark Paladin > Dark Avenger

The Dokkaebi’s Supernatural Stomp
Acquire Yaksha

Already Legendary

The Embodiment of Cold
Acquire Frozen Spirit

Frost Skull > Ancient Frost Skull > Frozen Soul

The Final Mystery of the Arts
Acquire Neo Samurai

Samurai > Neo Samurai

The Greatest Werewolf
Acquire Eternal Werewolf

Werewolf > Elder Werewolf > Alpha Werewolf > Eternal Werewolf

The King of the Labyrinth Ruins
Acquire Minotaurus III

Minotaurus > Minotaurus II > Minotaurus III

The Legendary Ninja
Acquire Master Ninja

Ninja > Ninja Master

The Legend of the Arena
Acquire Champion

Already Legendary

The Raging Wave
Acquire Poseidon

Water Skull > Aqua Skull > Poseidon

The Ruler of the Nether World
Acquire Grim Reaper

Already Legendary

The Unbreakable Pact
Acquire Devil Berserker

Berserker > Devil Berserker

Too Big to Be Called a Sword
Acquire Warlord

Warrior > Gladiator > Warlord

Ultimate Weapon: Bow
Acquire Sniper

Hunter > Bow Master > Sniper

Wanna Play a Game With Me?
Acquire Gambler

Already Legendary

War Fanatic
Acquire Carleon Commander

Carleon Recruit > Carleon Sergeant > Carleon Officer > Carleon Commander

White Mage
Acquire Sage

Mage > Grand Mage > Archmage > Sage

Roughly 6 skulls will already be at their legendary rarity but for most skulls will need you to visit Arachne to awaken/upgrade them, which will cost bones. Any skulls you don't want, you can destroyed for bones. (Excluding Skul's original skull)

Example: 　　　　
Werewolf > Elder Werewolf > Alpha Werewolf > Eternal Werewolf / Greatest Werewolf Achievement
Common > Rare > Unique > Legendary / Achievements 　　　　　　　　　 > = Awakens to

Arache can be found on these levels:
Floor 1-2, Floor 2-1, Floor 2-2, Floor 3-1, Floor 3-2, Floor 4-1, Floor 4-2, Floor 5-1

Skull upgrade costs:
Common to Rare: 10 bones
Rare to Unique: 30 bones
Unique to Legendary 100 bones

Excel link to Legendary skulls check list (also at bottom of guide)
https://www.dropbox.com/s/2v6assdptsiuu2w/Legendary%20Skulls%20ticklist.xlsx?dl=0

Work in Progress

For Tonight, We Dine in Hell!
Acquire Demi-god

Slave > > > Demi-god

God of Thunder
Acquire Thunderlord

Viking > > > Thunderlord

Hero of the Night Sky
Acquire the Absolute

Ascetic > Absolute

Strenght of 10,000 Men
Acquire God of War

Officer > General > God of War

Sword of Promised Victory
Acquire Unknown King

Already Legendary

1st Boss

Rescued Elder Ent
Defeat Yggdrasil, the Elder Ent

2nd Boss

The End of the Gold Mane Knights
Defeat the Leiana Sisters, the adjutants of the Golden Mane Knights

3rd Boss

False Goddess, Disappear
Defeat Chimaera, the distorted Goddess

4th Boss

The Fall of the Leonia Order
Defeat St. Joan ll, the Archbishop of the Leonia Order

Final Boss

The Final Battle of the First Hero
Defeat the First Hero, the Sword of Carleon


Quick Movements
Defeat Yggdrasil, the Elder Ent, without being hit


Master of Speed
Defeat the Leiana Sisters, the adjutants of the Golden Mane Knights, without being hit


Deadly Power
Defeat Chimaera, the distorted Goddess, without being hit


The Levitator
Defeat St. Joan ll, the Archbishop of the Leonia Order, without being hit


The God of Battle
Defeat the First Hero, the Sword of Carleon, without being hit

Dark Side of Harmonia
Enter the Dark Mirror to arrive in the Dark Side of Harmonia


New Start
Return after dying in the Dark Mirror


Broken World
First Dark Mirror Destroyed


Heart of Darkness
Acquire 100 Dark Quartz Hearts


Spider God's Protection
Offer 10 bones to the Arachne Clan's altar in one session

Dark Mirror, Black Market

All in
Use all chances to restock with Dark Quartz in the Black Market


Solid Career Choice
Acquire 2 additional inscriptions from the Black Market


Special Guest
Buy special items from collectors

Dark Mirror Abilities

Best Condition
Have 4 dark abilities in your inventory


Dark Ability
Acquire all dark abilities


The Cursed
Have 4 cursed dark abilities at the same time

Dark Mirror Inscriptions
Equipment has different inscriptions on them like fairy tale, fortress, poisonings, soar, strike etc
which gives you stackable effects; for example having one 'Absolute Zero' inscription will give Skul a 'normal attacks causes ice to erupt from the ground and freeze enemies' effect.

Hate the Sinner, Love the Sin
Have the 'Sin' inscription


Terrible Omen
Have the 'Omen' inscription

To get the achievements you need a piece of equipment with the sin or omen inscription. You can buy equipment from the black market or obtain equipment at the end of the level. Rescuing certain NPCs in cages can sometimes give you random equipment, like the evil druid and Little Gust & Lion Sword.

Witch's Help
Modify 5 different Inscriptions

Chapter 6

The War Begins
Reach Chapter 6 for the first time


Bone Specialist
Acquire Skul's Fragment 3 times


Dark-Tech Tank
Encounter the mysterious being


Hidden Existence
Find Enriched Dark Quartz for the first time


The Power of Science
Use the Dwarf Engineer's high speed transportation for the first time


A Worthwhile Adventure
Level 1 Dark Mirror Destroyed


Courageous Act
Level 2 Dark Mirror Destroyed


Energy and Perseverance
Level 3 Dark Mirror Destroyed


Self-confidence
Level 4 Dark Mirror Destroyed


Energy and Perseverance
Level 5 Dark Mirror Destroyed


Spirit of Challenge
Level 6 Dark Mirror Destroyed


Surprising Move
Level 7 Dark Mirror Destroyed


Phenomenal Success
Level 8 Dark Mirror Destroyed


Great Achievement
Level 9 Dark Mirror Destroyed


Nothing is Impossible
Level 10 Dark Mirror Destroyed

True Peace
Defeat Alexander, the Emperor of Carleon


True Salvation
Defeat Skul, The Dark Skeleton


The Strongest
Defeat the First Hero in the Dark Mirror without getting hit

.

Hall of Gods
Arrive at Hall of Gods

Playing the game, I can't always remember which Legendary Skulls I've equipped so instead of scrolling down my achievements, I thought a table on my second monitor would be useful.
So here's a Dropbox link where you can download tick list I made in Excel ;)

https://www.dropbox.com/s/2v6assdptsiuu2w/Legendary%20Skulls%20ticklist.xlsx?dl=0