The Shattered Greatsword is the heavy melee weapon that comes out of the Collapsed Kingdom in Legendary Monsters, a broken blade that hits for 12 damage and reaches a block further than a normal sword. The kingdom is the structure buried under the birch forests, and it comes with its own bosses and its own gear.
You won’t find one sitting in a 
Chest. It’s crafted from 
Metal Debris, which only drops from the armored minibosses sleeping inside the Kingdom, so the sword is a reward for clearing them rather than for looting the ruins. It swings once a second, which is slow enough that you’ll notice, though the damage per hit is the trade you’re making.
Obtaining
The Collapsed Kingdom generates under Birch Forest, Old Growth Birch Forest, Forest and Plains, and the first one might be a long ride from spawn. Most of its rooms hold Haunted Guards, Haunted Knights, Fractured and Fractured Apostles, and none of those drop Metal Debris. The two that matter are a Beheaded Knight and a Resurrected Knight, asleep together by the 
Somber Trapdoor.
Find a Collapsed Kingdom
Birch Forest, Forest or Plains
If your world has commands on, the locate command points you straight at one, otherwise you’re sweeping Birch Forest and Plains until you hit the ruins. It lands you at the gatehouse on the surface, and the knights are inside the tower behind it.
The first knight room isn’t deep, it sits in the tower a short way past the entrance, and there’s a second pair much further in, down the pit shaft and through the dungeon wings on the way to the Possessed Paladin‘s arena. That second pair is worth looking for, as two more Metal Debris covers the 
Resurrected Javelin as well.
Kill both knights
One Metal Debris each
Both knights start the fight asleep, and while they’re asleep they take no damage at all and can’t be given status effects, so a thrown Potion before you wake them is wasted. They wake when you come within about 10 blocks and take a couple of seconds to stand up.
Knight Drops
Somber Key Blade 
Somber Key Ring The Beheaded Knight has 195 health behind 14 armor and the Resurrected Knight has 190 behind 12, and while they’re within about 10 blocks of each other they fight as a duo, mostly taking turns rather than both swinging at once. Each one drops a single Metal Debris when it dies with no chance roll on it, so Looting does nothing for you here.
The exception to the turn taking is a paired attack they set up together on about a 15 second cooldown, which is the one moment both of them come at you at once. And if you break off, they start healing roughly three seconds after they lose you, 20 health at a time, so backing out to eat hands them most of the fight back.
They also carry a half of the 
Somber Key each, the Beheaded Knight dropping the Blade and the Resurrected Knight the Ring. Those two combine anywhere in a crafting grid into the Somber Key, with no shape needed, so you can make it on the spot.
Right-clicking the middle of the Somber Trapdoor with it opens the way deeper into the Kingdom. The key’s used up doing it, which is why each knight room comes with its own pair.
Craft the sword
2 Metal Debris, 2 Iron Nuggets, a Stick
It’s a shaped recipe of 2 Metal Debris, 2 
Iron Nuggets and a 
Stick. One pair of knights gives you exactly the 2 Metal Debris it asks for, so the sword and the Javelin can’t both come out of a single room, and you’ll need the second pair deeper in for the other one.
The nuggets are the easy part, as the Kingdom’s basic Chests turn up Iron Nuggets and 
Iron Ingots anyway. If you’re short, a 
Knight’s Sword Blade Half from any Haunted Knight turns into an Iron Ingot in the grid, which is 9 nuggets from a mob you’re already killing. The 
Long Stick Half a Haunted Guard drops does the same for the Stick, two per half.
Usage
In the hand it’s 12 attack damage at 1.0 attack speed, so you get one full-strength swing every second and the cooldown bar is what you’ll be watching. A 
Netherite Sword does 8 at 1.6, which comes out slightly ahead over a sustained fight, but it takes three hits where the Greatsword takes two. Against anything that closes, trades and backs off, fewer and heavier hits is the easier pattern.
It also adds a block of attack reach, taking you from 3 blocks to 4, and that’s the part that changes how fights feel. You can open on a mob before it’s in its own swing range, and back off a step and still connect. Sweeps work as on a vanilla sword, and with a second between swings the cooldown is nearly always full, so every hit sweeps.
Durability is 1500, and Iron Ingots are its repair material on an 
Anvil. Each ingot puts back a quarter of the bar, so four take it from empty to full, and Iron is common enough in the Kingdom’s own Chests that keeping it fed isn’t a problem.
Knight’s Sword
Shattered Greatsword
Resurrected Javelin
Netherite Sword
The 
Knight’s Sword is what you’ll have been using up to now, crafted from two Blade Halves and a Stick, and the jump from 6 damage to 12 is the whole reason to bother with the knight rooms. The Resurrected Javelin is the other thing Metal Debris makes, and it throws, which the Greatsword can’t. The tooltip’s lore line nods to where the scrap came from.
Damage, attack speed and reach each have their own multiplier under the “Shattered Great Sword” section of the common config, all set to 1.0 by default. Modpack makers can tune any of the three without touching the recipe.
Enchantments
It takes the full vanilla sword list and its enchantability is 15, the same as Netherite, so a level 30 table will give you decent rolls rather than the scraps a 
Diamond Sword tends to pull. Sharpness is the one that compounds hardest here, as a flat bonus on a 12 damage swing is worth more than the same bonus spread over a faster weapon.
Sweeping Edge is the other one that fits, because with a full second between swings nearly every hit into a group sweeps anyway. Mending finishes it off, since 1500 durability goes faster than it sounds on a weapon you’ll carry everywhere.
Tips
Wait for the cooldown bar to refill before every swing. Hitting early on a 1.0 attack speed weapon throws away a large slice of the damage and cancels the sweep with it, so the rhythm matters more on this sword than it does on anything faster you’ve carried.
Clear the Haunted Guards and Haunted Knights on the way in rather than running past them. The Haunted Knights drop Knight’s Sword Blade Halves and the Haunted Guards drop Long Stick Halves, which feed the rest of the Kingdom’s crafting, and a spare Blade Half covers your Iron Nuggets for the recipe.
Go in with a proper set of armor rather than whatever’s been scraped together along the way, as the knight duo will chew through even a good set. Have a way to repair or replace pieces afterward, since a rough pull between the two of them can leave a suit needing real work by the time both Metal Debris are on the ground.
Use the extra block of reach on everything, not just the mobs from this mod. Backing up a step mid-fight still leaves you connecting, and that’s the simplest way to play around a mob that hits harder than you do.
FAQ
It depends on whether you’re measuring a swing or a minute. The Greatsword lands 12 a hit against Netherite’s 8, so it kills most things in fewer swings, but Netherite’s 1.6 attack speed edges it on damage over time at 12.8 against 12. The extra block of reach and the Netherite Sword‘s 2031 durability pull in opposite directions, so plenty of players carry both.
Only from the Beheaded Knight and the Resurrected Knight, which both live in the Collapsed Kingdom. Each of them drops exactly 1 on death, with no chance roll and no Looting involved, so a cleared pair leaves you with the 2 the sword needs. It doesn’t generate in any Chest and it can’t be crafted, either.
No ability, no charged attack and nothing on right-click, it’s swings and sweeps like a vanilla sword with bigger numbers attached. The Resurrected Javelin is the one from this material that does something extra, as it can be thrown. If you want a weapon with an ability on the button, the 
Soul Great Sword and the 
Wand Of Clouds are the ones to look at instead.
Iron Ingots are its repair material, so an Anvil takes the wear back off, a quarter of the 1500 durability per ingot. Mending works on it as well, which is the easier answer long term, as you’ll stop having to think about it at all. Four ingots is a full bar from empty.
