The Stone-Sheathed Sword was one of the biggest mysteries during my initial playthrough of Elden Ring: Shadow of the Erdtree. You'll likely find this blade while scouring some catacombs and get excited when you pick it up, realizing it's a legendary weapon. Sadly, it sucks. It doesn't have a skill or good scaling, but the description explains that it's a “relic of a lost civilization” that “served a purpose at altars of light and darkness”. 

My first thought was that it had some kind of narrative purpose, but the good news is that you can draw the Stone-Sheathed Sword from its scabbard and turn it into a real legendary weapon. So, here's how to use the Stone-Sheathed Sword and where you can find those aforementioned altars of light and darkness. 

Where to find the Stone-Sheathed Sword

Now, before we start, it's worth noting that you can find the Stone-Sheathed Sword at any of the three altars mentioned in this guide. Doing so will transform the remaining two altars into those required to change the weapon's form. However, doing it the way I mention below is the earliest opportunity to grab the weapon. Just remember the above in case you accidentally find it later and are coming here to work out what's going on.

The earliest location you can get the Stone-Sheathed Sword is in the Fog Rift Catacombs just north of the Castle Front site of grace in front of Castle Ensis in Gravesite Plain. You'll have to run a gauntlet of fire pot-throwing soldiers to reach a foggy area with coffins and statues and the dungeon entrance at the far end. Now, progress through the dungeon until you reach the long room with the spiked ceiling traps above and a wizard on a balcony at the far end firing spells at you.

Make your way to where the wizard is—you can use the alcoves to avoid the ceiling catching you when it slams down—and then stand on the balcony railing to bait the ceiling trap down and jump into the revealed space. The trap will raise you to the ceiling, where you can jump off to find a chest containing Ash of War: Blink Bolt. Now turn around and run along the top of those ceiling traps to find a little space on the far right containing a mysterious altar with the Stone-Sheathed Sword.

How to get the Sword of Darkness

Now you've got the blade in question, what do you do with it? It's a legendary weapon, alright, but it straight up sucks in its current form. What you need to do is to draw the blade at one of two altars of light and dark to transform it into one of two possible legendaries. The cool thing about this weapon is that revisiting the altars also lets you keep alternating between whichever version you want. 

The first altar is at the Unte Ruins in lower Scadu Altus. To get there, you'll need to take the main path through the Shadow Keep until you reach the courtyard with the flaming boats. Climb down the ladder on the east side, head behind the waterfall, and continue to the room with the painting. Here, you'll find an illusory wall on the right side, and after it, a coffin you can climb into that takes you down to the Castle Watering Hole site of grace and the Unte Ruins straight after.

Before you can access the ruins, you're going to have to kill two Furnace Golems; one hanging around near the entrance, and another that's sitting down, blocking your access into the ruins themselves. In order to wake the second golem up, you need to toss a Hefty Furnace Pot into its brazier from the balcony next to it—check our guide linked above if you want the cookbook location and crafting mats for that. Beat the golem and you'll find a flowery courtyard with an altar on the right side that looks suspiciously similar to the one you got the Stone-Sheathed Sword from.

This is the altar of dark and raising the sword here will transform it into the Sword of Darkness shown above. It's a faith-scaling straight sword with holy damage and the Darkness skill, which deals holy damage and reduces holy damage negation in an AoE.

How to get the Sword of Light

You can find the second altar in the Ancient Ruins of Rauh area. You can access this via the western rampart of the Shadow Keep in Scadu Altus, where a bridge leads towards the region. If you haven't been here before, I'd suggest you grab the map fragment from its lower level, since that'll make finding this spot easier. Progress through the region until you reach the Rauh Ancient Ruins, West site of grace. From here:

• Head up the stairs past the sorcerers with the slinky magic and then turn left in the big room to find an elevator you can take up
• Ride through the watery glade with the Hornsent warrior and enter the scarlet rot-infested ruin on the far side of the stone gazebo structure
• Turn right immediately once inside and take the elevator down before turning left and following the corridor into a long room filled with pests and a statue of a woman at the far end
• Go to the far end of the room and exit outside to spot a big stone tower ahead of you and a broken bridge leading to it

If you've got good eyesight, you'll probably spy the altar of light directly ahead of you inside the tower. The good news is that there's an invisible bridge leading over there—just run straight across to the altar and raise the Stone-Sheathed Sword—or the Sword of Darkness—to turn it into the Sword of Light. As before, it's a holy damage weapon, but now it has the Light skill that fires off rays of light in an AoE and boosts your holy damage temporarily. 

Remember, you can swap between both legendaries by visiting their respective altar again and raising the sword once more.