You can unlock the secret area by going through the game and coming back in new game plus
If you go to Tartarus at the New Moon, your chances of getting one of the random effects at Tartarus (more experience, drop rate, pitch black dungeon, red or golden shadows only, more boxes, facing death, etc) will get higher.
Most of the time, you'll probably be facing the final boss with the best equipment Officer Kurosawa can offer you, however there are much better weapons out for grabs and that's not all; you don't get them by stumbling upon a gold chest in Tartarus out of sheer dumb luck, you can get them just by doing a little business with the antique store a few feet to the right of the police box!
Of course, you'll need to get specific personae and Nihil/Void weapons from golden shadows but they're worth every penny and second you pitch in to this, They have a whopping 450 attack rating, 100 accuracy points and if that wasn't good enough, they also bestow 10 points to ALL of your stats!
Here's a list:
1 Handed Sword: Lucifer (Judgement) - Lucifer' Blade
Skadi (Empress) - Snow Queen's Whip
Bow: Chi You (Tower) - Quintessence Bow
2 Handed Sword: Masakado (Tower) - Masakado's Katana
Gloves: Mara (Tower) - Evil Gloves
Gun: Metatron (Aeon) - Metatronius
Spear: Shiva (Tower) - Pinaka
Knife: Asura (Sun) - Vajra
Axe: Beelzebub (Devil) - Corpse Rod
Naginata: Kartikeya (Star) - Vel Vel Muruga
PS: Attempting to sell these weapons to Kurosawa yields only 2 yen, Do I detect jealousy from Kurosawa to your new supplier?
To access Monad, you must first unlock "Heaven's Door" at the highest level of Tartarus (floor 254). Accomplish this by first accepting and complete Request #53 for Elizabeth, "Retrieve 1 Bloody Button." This requests requires you to lure out and defeat the Reaper on any floor in the dungeon.
Then return to Elizabeth when you are finished to receive your reward, and open the way to Monad! This also unlocks Requests #55 and #72.