How to Reach the Unte Ruins
You must use a covert route from Shadow Keep to reach these ruins, which are located immediately southeast of the fortress.
If you are able, go straight through the steel doors from the Main Gate into Grace. If you haven't discovered this shortcut, just play the dungeon as usual until you come across the blazing boats, which are unique to this section and will serve as a significant marker.
Examine the picture for a cunning ladder that you must climb before going under the waterfall and taking aim at the false wall that is shown in the next picture:
As usual, you will be placed in a coffin at the base of the aqueduct, concealed behind the false wall. The next chamber, which is shown on the map below and faces the Unte Ruins, is where you'll exit:
Next, Hefty Furnace Pots are required.
First things first: confirm that you own The Greater Potentate's Cookbook [2]. This is located in the Traveler's Rest, which is run down below.
Should you be able to Make Large Furnace Pots already, excellent! Now let's proceed.
A Furnace Golem is obstructing the path into the Ruins (I've already dealt with him in my picture, so use your imagination), so to wake him up, stand in the designated position and toss a Hefty Furnace Pot into his skull: 
The Furnace Golem will then rise, and you have the option of either defeating him or entering covertly.
Loot from Unte Ruins
Within the Unte Ruins is the Giant Golden Arc Incantation and the stone slab required to craft the Sword of Light. There's an invisible bridge that connects the distant Rauh Ancient Ruins to the sword that you must lay on the slab.
The Incantation is a wide-arc projectile that is launched directly from the player and costs 34 Faith. Another straight sword with a faith motif and a distinctive weapon art is the Sword of Light. Priests and warriors, exult! Everyone else. Well, exploring was enjoyable, wasn't it?