Mjolnir Underground Cave (250+) is part of the new level 250 dungeons, here you can obtain materials for the creation of Flush Weapons.

Mjolnir Underground Cave (250+)

Rebellion Crew /navi ein_fild08 192/49 NPC Warps you to Mjolnir Underground Cave.	Daily quest rewards 1x/5x Myornil's Treasure or Myornil's Energy Weekly quest rewards 10x Myornil's Treasure or Myornil's Energy
Items obtained via map drop.	Myornil's Treasure Myornil's Energy Zelunium Zelunium Ore Shadowdecon Rough Shadowdecon Etel Dust

Monsters

Monster	Health	Race/Element/Size	Cards
Punch Bug	48.836.035	Insect/Earth3/Medium	ATK +5% _______________________ When compounded on a Knuckle class weapon: Increases Melee Physical Damage by 10%. For each 3 Refine Levels: Increases Melee Physical Damage by additional 2%. Base Level at least 200: For each 3 Refine Levels: Increases Melee Physical Damage by additional 3%. _______________________ Type: Card Compound on: Weapon Weight: 1
Aferde	47.874.030	Brute/Holy1/Small	Increases Ranged Physical Damage by 6%. _______________________ When equipped with Timbers Card: Increases Ranged Physical Damage by additional 8%. _______________________ Type: Card Compound on: Accessory(Left) Weight: 1
Dispol	50.482.947	Insect/Wind2/Large	Increases Magical Damage with Holy element by 35%. Decreases SP Consumption by 10%. _______________________ Type: Card Compound on: Accessory(Left) Weight: 1
Timbers	45.319.714	Plant/Neutral4/Medium	Increases Ranged Physical Damage by 6%. _______________________ Type: Card Compound on: Accessory(Right) Weight: 1
Renire		Insect/Water3/Medium	Increases SP Consumption by 10%. _______________________ For each Refine Level: Increases Magical Damage with Wind element by 3%. _______________________ Type: Card Compound on: Shoes Weight: 1

Flush Weapons

Flush Weapons you can get from Grade Enhancer by looking for Blacksmith Sinea, you can use /navi grademk 47/184 or go to Warper choose Special Areas and then Grade Enhancer, the required items vary depending on the weapon you are reforming.

Flush Weapons Refine

To refine these weapons using the Reform System, you can do it again with Blacksmith Sinea, remember that the materials will depend on the weapon you are going to refine.