Shadow of the Erdtree's legendary weapons feel even more inventive than a lot of those in Elden Ring's base game. As far as I know, there are four of these weapons, and while one of them is kind of a copy-paste of another weapon we already have, two of them have their own entirely unique, inventive mechanics. The Sword of Dark/Light, for example, is a weapon whose form you can change by visiting certain locations in the Land of Shadow.

There's also Euporia, a fancy-looking twinblade that charges and begins glowing as you hit enemies, empowering it to deal extra damage. Two of Erdtree's legendary weapons are faith-scaling, one scales with intelligence, and one with arcane, so they might not fit your build. Still, they're cool to own regardless. Here are all of the Shadow of the Erdtree legendary armaments I've found so far, plus how to get each. 

Sword of Darkness/Sword of Light (Straight Sword)

• Stat requirements: 14 Str, 11 Dex, 24 Fai
• Base scaling: Str D, Dex E, Faith E
• Weapon skill: Darkness/Light

This legendary weapon is a really cool one; a straight sword that can change its form as many times as you want by visiting certain altars located in the Land of Shadow. While I covered the altar locations and the specifics in the Stone-Sheathed Sword guide—the name of the weapon when you first find it—you can grab it inside the Fog Rift Catacombs in Gravesite Plain, to the north of the soldier camp outside Castle Ensis. It's pretty well tucked away, so be sure to see that guide for the exact steps on transforming it.

In both forms it's a faith-scaling straight sword that deals holy damage, but changing its form changes its skill. The Sword of Darkness conjures a circular AoE of black fog that deals holy damage and reduces holy damage negation, while the Sword of Light fires out rays of bright light, dealing holy damage and boosting the holy damage of the sword.

Ancient Meteoric Ore Greatsword (Colossal Sword)

• Stat requirements: 35 Str, 10 Dex, 15 Int
• Base scaling: Str D, Dex E, Arcane D
• Weapon skill: White Light Charge

This colossal greatsword is the legendary weapon linked with Shadow of the Erdtree's new Forge dungeon type. To get this one, you'll need to progress through the Ruined Forge of Starfall past in Scadu Altus until you lower the gate at the end. You'll then need to pull the lever on the other side, and jump on top of the gate to rise up and find the furnace with this weapon.

It's moveset is pretty standard for a colossal sword, but it has a fun skill called White Light Charge that lets you zoom forward quite a distance and unleash a powerful thrust.

Velvet Sword of St. Trina (Straight Sword)

• Stat requirements: 10 Str, 12 Dex, 14 Int
• Base scaling: Str E, Dex D, Int E
• Weapon skill: Mists of Eternal Sleep

This legendary weapon is extremely similar to the Sword of St. Trina from the base game. In fact, it's identical except for the fact that it deals the new eternal sleep debuff instead of regular sleep. This upgraded version of the sleep status effect is only available on two weapons in the new expansion as far as I can tell, the other of which is Thiollier's Hidden Needle, which you get by completing his quest. 

In terms of where to find this weapon, you'll need to descend into the Stone Coffin Fissure at the tip of the southernmost peninsula in the Southern Shore region. Access to this won't unlock until you reach the Shadow Keep. Since this location is part of Thiollier's quest, I recommend looking at that guide for exact details, especially if you want to do it at the same time. 

Once you arrive at Fissure Cross site of grace, head left to the big sloped stone coffin and make your way down the front of it. Drop down a couple of times and head under another stone coffin to see a small cave with the sword inside.

Euporia (Twinblade)

• Stat requirements: 16 Str, 16 Dex, 24 Fai
• Base scaling: Str D, Dex D, Fai D
• Weapon skill: Euporia Vortex

This legendary twinblade is perhaps my favourite of the new weapons. It has a special mechanic where you charge its blades till they're glowing by dealing damage, and then you can unleash the blade in front of you, spinning it like a helicopter rotor.

The only problem is it isn't accessible until you're right at the end of the expansion. If you're not there yet, I'd suggest completing it, and then coming back here to find out the exact route. I don't want to spoil anything, but this one is very hard to find.

What, still here? Then buckle up. From the Spiral Rise site of grace in Enir-Ilim:

• Head down the stairs, past where you can see the Hornsent with his arms outstretched and peer over the right railing of the staircase to see a corpse on a set of stairs you can drop to below
• Climb the stairs, look for the broken railing, and drop down onto the ledges below
• Descend the ledges all the way down and around the far corner to spot a tower with gargoyles you can drop to below
• Follow the sloping tower ledge to find another staircase you can drop down to. Follow the stairs down to the corpse at the bottom
• Now for the tricky final step: off to your right is a window. Get a run up and jump through this to find yourself back in Belurat, Tower Settlement

From here it's plain-sailing down the big elevator, though you will have to fight another of those storm-calling lion men en route to the treasure room with the twinblade. If you're having trouble, you can always hide next to the doorway and wait for him to pass.

When you reach the treasure room, you might recognise it at the mysterious bridge that's just above the poison section in Belurat you can open using the Well Depths Key.