While there are plenty of subweapons to choose from in Aeterna Noctis, the Scythe is hands-down one of the best and available fairly early in the game. However, like all subweapons, you need to trek a fair amount through a dungeon to find this handy weapon.
First, you will have needed to clear the Tower of Light and defeat the boss, The Queen’s Guardian (otherwise known as that annoying sword boss.) Next, head to Central Square, which is west of the Old Man at the Crossroads. You must enter Central Square from the East to unlock this dungeon. Progress through the burning area until you come across a Light Switch and a pit. Open the switch and jump into the pit to access the Ancient Catacombs.
From here, you will have to deal with plenty of Slime and flying enemies, in addition to Knights and Archers, so make sure you’re avoiding damage whenever possible. Follow the route outlined in red to locate the Scythe. You will have to clear a few waves of enemies before you can leave, but the Scythe makes quick work of them.
A note about the Scythe — each hit generates 1 Blood Orb, in addition to whatever Blood drops from the enemies when they die. This makes the Scythe a very potent weapon, as it allows you to build Blood Meter for healing or using the Axe much more often. As soon as you clear Tower of Light, you should unlock this fantastic weapon.