Below the graveyard south of Ramoria. Below the Musician there is a bridge of dirt, it takes you to a large green portal surrounded by white columns.
How to get in
If you have the Key of the One in your hero's inventory you will be allowed in. Alternatively because you must go to the Temple a number of times for the Good Main Quest, you can get in if you have a thievery level of 20+.
Inside the Temple
Once inside you can go either up or right. To the right will lead to a room with a Magic Box in the center. If you're holding the Key of the One and move next to the Magic Box the box will break and the Key of the One will be consumed, in its place will be an Amulet of the One. Amulet of the One, when used, will teleport you to the Temple of the One.
Going up from the entrance will take you to another portal which in turn will transport you to a large room surrounded by portals. Each portal will be surrounded by different terrain which will signify where it will take you. Next to the portal that will take you to the portal room will be a floating vault called the Temple Vault. You can buy items from here for money.
There are 15 portals in this room. The top three (going clockwise) will by linked by tiles similar to those in Fira and Alexandria , which will take you to Fira, Ramoria and Alexandria (Good cities 1, 2 and 3). Then there is a portal with trees behind it which will take you to the Tree of Gods. The next will be surrounded by coral and will take you to a sea area WestNorthWest of Fira outside a crypt. The next portal is surrounded by lava and will take you to below the Ramoria Graveyard where the terrain changes to volcanic. The next two portals are surrounded by Snow then Ice and will take you to the Mountain area covered in snow and ice north of Fira. After those portals the next is the portal and the very bottom of the room, also surrounded by lava but will take you right outside the portal to Hell. The next three portals, surrounded by Tombstones, skeletons and then coffins, will take you to the Ramorian Graveyard, outside the entrance to the Black Citadel and outside the boss door in the Vampires Den. The next portal is surrounded by desert sand and will take you to the desert just below the entrance to Darkhaven (evil Bandit Camp). The portal after Darkhaven, with a sunken ship in it, will take you to Pirate Fortress(evil Fortress). The final portal, left of Fira and similarly to the Tree of Gods portal surrounded by trees, will take you to the forest above where the God of Nature can be summoned.
In short the portals (clockwise and starting from the very top) will take you to: Ramoria Town, Alexandria City, Tree of Gods, Ocean, Volcano, Snowy Mountain, Icy Mountain, Hell, Ramoria Graveyard, Black Citadel Entrance, Vampires Den, Darkhaven, Pirate Fortress, Forest and Fira City.
Returning to Temple
If one wishes to return the the Temple of the One, you may get in as before with the Key of the One or with thievery level 20+. Otherwise you can use the Amulet of the one which when used will teleport you to below the portal that takes you to the portal room in the Temple. Amulet of the One, Red Amulet of the One, Green Amulet of the One, Blue Amulet of the One and Prismatic Amulet of the One will all take you to the Temple of the One.