In order to get to the Abyss and battle the final boss, you need to clear the Veo Lu Sluice during the 5th year of the game. Doing so allows you access to Jegon River and Lynari Desert, where you need to get the Unknown Element to upgrade your Crystal Chalice. With this, you can unlock all areas of the game on your 5th year.
The deadly miasma that plagues the land prevents you from advancing further. However, as the years go by, the more areas and locations become accessible. In order to access all locations, you need to have the Unknown Element which can only be unlocked from the 5th year onwards.