From the NPC Uncle Kaga -
It is said that 12 years ago, in order to expand their power, the Eye of Wisdom targeted the resource rich Howling Mountains, launching a magical eradication campaign to clear out those dangerous monsters that threatened the humans in the region.
The most powerful monster at that time was Pirlanok, who called on the earth spirits, goblins, and various other native creatures of the Howling Mountains to resist, Hodu Hammertooth and Bonebreaker answered the call, adding two extremely powerful armies to his cause. Of course, this led to a very terrible and desperate battle.
Ultimately, the Eye of Wisdom manage to seal Pirlanok into the Dead Tree Cave and forced the remnants of Hodu Hammertooth and Bonebreaker's armies to flee into the Windmill Basement - Where the Eye of Wisdom had prepared powerful magical seals and sacred vessels."
Level 20+ is recommended with a strong tank and speedy healer to take down the powerful boss mobs inside.
 Getting Into the Windmill Basement
To gain entrance to the windmill, you will require a Gloomy Vapor Stone which are obtained from the Golo creatures in the Dead Tree Cave and the Barren Caves.
Once in possession of an Gloomy Vapor Stone speak with Uncle Kaga standing in logar by the Windmill entrance.
Speak to him and choose the option Enter the Windmill Basement which in turn will bring you to another menu where you need to choose Creating a Vapor Stone.
Once done, one Gloomy Vapor Stone from your inventory will become a Dazzling Vapor Stone, this is required for entrance into the Windmill Basement and will last in your inventory for one hour or until thrown away.
 Traversing the Windmill Basement
Once inside the instance, you will find yourself in an empty room with a hallway leading out of it where the first monsters are located. In the first room where monsters are, there is a special monster named Gomio. Be careful of this monster, even at higher levels, if Gomio manages to cast all of his magic, all of your party members can suffer major damage or death.
While exploring this dungeon, you will come across rooms at the end of most hallways with the standard assortment of monsters in them, but there is one you will find that has a large box called Box that spreads Wail. This box, if opened will deliver either three standard dungeon drops to your inventory or one boss drop with one standard drop. At the same time, it will spawn an assortment of normal monsters to ambush you.
At nearly 2/3 of the way through the instance, you will encounter the primary boss of the dungeon Hodu Hammertooth. His normal attacks are nothing important, but he has an AoE skill where he will shout "Burn!! (Ancient Language) An'Sola Shale" that will inflict massive damage to everyone nearby. This attack is very powerful even for high level players, if you are not very high level, this skill can be interrupted. Sometime after Hodu reaches less than 50% of his Hp, he will retreat taunting you and telling you that you will have to find him.
To find Hodu again, you must locate The Spirit of Dead Hero who spawns in one of four rooms elsewhere in the dungeon. Those rooms could be any of the three rooms other than the one with the Box that spreads Wail passed on the way to where you found Hodu or the one just beyond there with purple stonework.
Once you have located The Spirit of Dead Hero you must click on him to activate him.
One of three outcomes will randomly occur at this point:
1.) Hodu Hammertooth will spawn and attack you, The Spirit of Dead Hero will cast the buff Fighter's Blessing on you and disappear, leaving you to kill Hodu.
2.) Hodu Hammertooth will spawn and shout "Serve me! (Ancient Language) An' Sola Vie' Ta" turning The Spirit of Dead Hero into Geianth, a second boss, and they will both attack you.
3.) Hodu Hammertooth will spawn and attack you, The Spirit of Dead Hero will become Ghost of the Fallen Hero and say that he is there to help you and then requests healing.
When Hodu Hammertooth is killed, he leaves a chest called Drakor's Treasure for you to loot, Geianth also leaves a chest, named Finely crafted Box, and if Ghost of the Fallen Hero made it through the battle, he will also have boss drops for you. Hodu and Geianth both carry a random piece of the tier 1 windmill armor, up to three of the crystals required to get level 25 elite skills, and a tier 1 enhancement rune. Also, there is a chance that one of them will have a Rune Crystal which is a very rare drop used in some of the better recipes for the upgraded windmill armor sets.
Once you have accomplished your goals with the boss, you may choose to go down the last side-hallway on the way out to find two purple treasure chests which contain random low-level harvest materials, production runes, and low-level healing/magic potions.
Once done, the portal out is at the end of the main corridor and will take you back to Logar, where you can obtain recipes for using the materials you may have picked up on your run through the windmill from Uncle Kaga.
for Windmill Basement
 Standard Monsters
 Purple Treasure Chest
 Boss Standard Drops
 Boss Armor Drops
This table is available on a separate page here.
These charts are available on a seperate page here.