SWORD ART ONLINE Re: Hollow Fragment

You need do all the hollow Missions ones. There are total 80 hollow mission from the main game. You don't need do DLC hollow mission.

You can do the hollow mission in Multiplayer and Single player.

Mission in list with >>> means it will unlock next hollow mission in Single player
The "The nightmare king’s Legacy" mission is a Single Player only mission

Here is the list hollow missions i did.
Use it as a checklist

Selvendys Forest Area.
1. The Shadow-dewelling beast
2. Lust of the Young Kobold lord
3. Assault of the golblin Thieves
4. Search fort he Beasts’ Altar >>>
5. Report on Location of Altar
6. Her majesty Buzzeth
7. A flower For his grave
8. Love is Light
9. Necessary Deforestation
10. The famished Invaders
11. At the mercy of Poison Mushrooms
12. Black-Winged Assailants
13. Deadly Intent From Alevaste
14. Goblin Wiles
15. Devourer of the Darkness
16. Bane of the Forest
17. Corrosive Substance (DLC)
18. Uproar of the Palladions
19. Prime cuts
20. Cries for cleansing
21. Honey on the brainstem
22. In the Pursuit of truth
23. Dracinian Revelry
24. Infestation from the Deep Wood (DLC)

Bastia Gate Floating Ruins Area
1. The Sky master’s Sword
2. Beastly Curse >>>
3. Order of Cursed Knights >>>
4. Beastly Intent
5. Lost Reaper From the abyss
6. Dark Demon Eviction
7. Rightful Owner of the Chest
8. Secret Garden
9. A Nest in Need
10. Sky-High Wanderlust
11. Baby Dragon Boom
12. Pruning Job
13. Soul-Crushing Songstresses
14. Underground Infestation
15. The Loony Alchemist: Investigation >>>
16. The Loony Alchemist: Plot
17. The Glory of the Streak
18. Sentinel of the Gaint chest

Graceleaf Bay Area
1. Carapace Metamorphosis
2. The All-Seeing Eye
3. Aquatic Ruler
4. Invading Army from the Sea
5. Scorpion King of the Mine
6. Echo of Resentment
7. He Who Would Rule Graceleaf
8. Return of the Conqueror (DLC)
9. Spawning along the Water Edge
10. Council of the Three Beasts >>>
11. This Council Disbanded
12. Gourmet Crab Entrails
13. Immortal Band of Pirates
14. The Immortal Pirate Captain >>>
15. The Captain’s True Form
16. A Place Fit for a Dragon
17. Immortal Pirates: Captain of War
18. Lost mine >>>
19. Escape From the Abandoned Mine
20. Immortal Pirates: Scheming Seaman

Driogeer Skycaves Area
1. The Soulless Reaper
2. Blood-Starved Hellhounds
3. A Change in Tastes
4. Gaint Flower Eyes
5. To see a forest Burn
6. Assault of the Blade Dragon
7. The uninvited Kobold Adventurer
8. The Revolt That Wan’t to Be
9. A Knight in Search of His Leader
10. End of the Road for a Traitor
11. Justice without Fail-safes
12. Greasy Movements
13. Six Sentient Treasures
14. Supreme Life
15. The Guardains’ Deadly Duty >>>
16. Supreme Guardain Assembled!
17. Twin Mechanical Knights
18. A Genocidal Emperor’s Rebirth

Alevaste Otherworld Area
1. Purple Crystal of Disaster
2. Chaos-Fueled Scorpions
3. Born From The Nightmare of Chaos
4. Left Arm of the Nightmare King
5. Eye of the Nightmare King>>>
6. Other Eye of the Nightmare King
7. From Within the Cracks
8. Grugdge-Steeped Guillotine >>>
9. Resting Place for Nihilism
10. Headhunter’s Sickle
11. Grasp of the nightmare King >>>
12. The Nightmare Continues…
13. Jaw of the Nightmare King
14. Spellbound by a Sword
15. Ruin-Gnawing Ants >>>
16. Attack of the Female Warrior Ants
17. Release to Oblivion
18. The nightmare king’s Legacy (Single Player only)

Abandoned Area ( DLC)
1. Otherworldy Swordsmanship
2. Prelude to Eruption
3. Despot of Tremors
4. (Advanced) Blue Lave Incarnate
5. (Advanced) Tremor King xx
6. (Advanced) Alpha Blade Dragon
7. (Advanced) A Soul on Fire
8. (Advanced) Reborn of Flames
9. (Advanced) Queen of the Snowcap
10. (Advanced) Requiem of Death
11. (Advanced) The White Empress
12. (Advanced) Dragon of infinite Might
13. Boiled Crab Banquet
14. Giant Crimson Goo
15. Food Coma
16. Protected Species
17. Victims of the Flames
18. A Tense Situation
19. At the Center of Destruction
20. Fury of a Guardain God
21. Assault of the Demons
22. Raging River of Fire
23. Molten Master
24. The Return of Despair
25. Calamity Born from Despair
26. Mispaced Hot-Bloodedness
27. Cool, Calm, and Run for it!
28. Frozen River of Blood
29. Frozen Primeval Fiend
30. A Demon’s Scheme
31. A lesson in Hunting
32. Spirits of Overwhelming Sadness
33. Holo to the Otherworld
34. Conquerors of the Cold
35. Golem Upsurge
36. The Transient’s Power Grab
37. The Return of Calamity
38. Calamity-Born Despair
39. Mind-Controlled Might
40. The Knights Who Fel lto the Darkness
41. A Shadow among Shadows
42. The Truth Will Come Out
43. Playing God
44. Best Laid Plans o’ Monsters and Men
45. Undercover Investigation
46. The Science of Violence
47. Classified Flowers

Good luck with doing all the Hollow missions