Part of the Dark Spirit Weapons Set Quest
Specter Palace 
Skills: Demon Spider (Hidden Boss)
|1||Sticky Thread||Piercing attack on all enemies + SPD down S (3 turns)|
|2||Shock Trident||Earth-type physical attack on all enemies + stun|
|3||Sturdy Thread||Earth-type piercing attack on one enemy + self PWR +20% S (4 turns, max 10 stacks: +110%)|
|4||Shock Trident||Earth-type physical attack on all enemies + stun|
You must have the Curse Breaking Flint to initiate the fight ("Nightmare Nab" rumor). To access the boss's location shrink yourself with the Miracle Water and then go down/south and enter the mousehole on the right. Head to the topmost crevice and use the Flint on the spiderweb there.