The Catacombs is a large underground dungeon in Iron’s Spells ‘n Spellbooks, buried under most Overworld biomes. It’s a maze of crypts, hallways and storage rooms full of Catacombs Zombies, Skeletons and Necromancers, and right at the center of it is the throne room where the Dead King lies waiting. Clearing it out is the only way to get to that fight.
There’s plenty to loot even if you never wake the Dead King up. The coffins and wall Chests hold Scrolls and 
Arcane Essence, there’s a 
Bone Vault in the throne room, and the 
Magic Spawners in the halls will send waves of Catacombs Zombies at you for a reward.
Finding the Catacombs
The Catacombs doesn’t show up on any vanilla map, so the best way to find one is with a 
Wayward Compass. It points to the nearest Catacombs as soon as it lands in your inventory. You craft it from four Arcane Essence in the corners, four 
Iron Ingots on the sides and a 
Soul Lantern in the middle, so it’s cheap enough to make early on.
If the compass doesn’t have a location stored yet, right-clicking it tries to find one again. Either way you’ll need to dig down once you’re standing above it, as the whole structure is buried and there’s no entrance on the surface.
The Wayward Compass only ever points at the Catacombs, so it won’t help with the mod’s other structures. If you’ve got cheats on and just want the coordinates, the locate command does the same job without any crafting, which is a quick way of checking how far away the nearest one is.
The dungeon’s starting room generates somewhere between Y -40 and Y 20, so it can be just below the surface or deep in the deepslate layer. It’s allowed in a huge list of biomes, including every forest, taiga, jungle and hill biome, plus plains, deserts, savannas, swamps and even Dripstone and Lush Caves, so you won’t need to go anywhere unusual to find one.
They’re spread out quite a bit, with the game trying to place one roughly every 72 chunks, so the nearest one could be a fair walk from spawn. That’s why the Wayward Compass is so useful, as it saves you digging around at random. Badlands, Mushroom Fields and oceans aren’t on the list, so you won’t find one under those.
Layout
The Catacombs is built out of randomly picked rooms and hallways around a central throne room, so no two are the same, and it can stretch up to 96 blocks out from the middle. Hallways come in straight sections, T junctions, turns, spiral staircases and stairways, and some of the straight hallways are trapped, so it’s worth watching where you step.
The rooms are a mix of burial chambers, living quarters and a few nastier surprises. It’s easy to get turned around, so picking one direction at every junction and sticking to it is a good way to make sure you don’t keep walking through the same rooms.
Inhabitants
The Catacombs has its own spawn list instead of the normal underground mobs. Catacombs Zombies make up most of what spawns inside, with Skeletons about half as common and the occasional Necromancer. Catacombs Zombies are normal Zombies with a random set of Leather, Chainmail or Iron armor that never drops.
One in five Catacombs Zombies also spawns with a permanent buff, which can be Invisibility, Speed II, Resistance II or Strength. The invisible ones are the most annoying, as you’ll only really see their armor floating towards you in the dark. The Necromancer only has 25 health, but it casts spells from a distance and raises extra undead, so it’s usually the one to kill first.
The Catacombs is completely underground and most of it is dark, so more Catacombs Zombies will spawn around you, and the invisible ones are harder to spot. Lighting up rooms as you clear them leaves a trail behind you as well, which makes it easier to find your way back out.
The Dead King
The Dead King’s corpse sits on a throne in the middle of the Catacombs. It doesn’t do anything until a player right-clicks it, or hits it while standing within 8 blocks, and then it slowly rises and turns into a 500 health boss with 15 armor. You can loot the whole dungeon without ever waking him up if you want to.
His phases, his attacks and the best gear to bring are all in How to Find & Beat the Dead King in Iron’s Spells ‘n Spellbooks. Once he’s been beaten, he leaves a Dead King Soul behind on the throne, which is how he can be fought again.
To bring him back, right-click the soul with a 
Dead King Phylactery, which you craft from eight 
Dead King Phylactery Shards around a Soul Lantern. You’ll find shards in the Bone Vault and from the Magic Spawners in the halls. Every player who takes part in the fight also gets a 
Bone Key when he dies, which opens the Bone Vault in the throne room.
The Dead King rises right in the middle of the throne room, and any mobs you’ve left alive nearby will join in. Clear the room out first, drink your potions and make sure you’ve got some way to heal before you right-click the corpse, because once he’s up there’s no way to put him back to sleep.
Ominous Catacombs
The Catacombs has an Ominous version as well. If anyone within 24 blocks has Bad Omen when the Dead King rises, their Bad Omen turns into Trial Omen and he rises as an Ominous Dead King. Each level of Bad Omen gives you 15 minutes of Trial Omen, and while you have it the Magic Spawners switch to their Ominous rewards too.
The Ominous Dead King deals 20% more damage, moves 10% faster, his summons hit 50% harder and he gets an extra 30 armor. On top of his normal drops he has a 50% chance of the 
Wicked Bone Ring, which lets spell projectiles bounce once more, a 70% chance of a Fang Swirl Scroll, which can’t be had any other way, and a 
Tincture of Forgetfulness for every player.
The easiest way to get Bad Omen is an 
Ominous Bottle, which gives you 30 minutes of it. Every Magic Spawner in the halls has a 15% chance of dropping one when you beat it, and they also turn up in the rampart Chests of the Lost Citadel. The Tincture of Forgetfulness resets the Bone Vault, so you can open it again after a second fight.
Loot
Most of the loot in the Catacombs comes from coffins, wall Chests and pots, with the Bone Vault in the throne room holding the best of it. The Magic Spawners in the larger rooms also give out rewards when you beat the waves of Catacombs Zombies they send at you, and those rewards are where most of the Phylactery Shards come from.
The vault sits in the throne room beside the Dead King, and it stays locked until someone has a Bone Key to use on it. It’s the one container in the Catacombs that every player gets their own loot from, rather than sharing what’s inside.
Bone Vault
Dead King Phylactery Shard 
Golden Apple 
Enchanted Golden Apple 
Blood Vial 
Ancient Knowledge Fragment 
Angel Wings Scroll 
Diamond 
Arcane Ingot The Bone Vault always gives you one to three Phylactery Shards and either a Golden Apple or, one time in four, an Enchanted Golden Apple. It also has a good chance of an Ancient Knowledge Fragment, a couple of Scrolls or 
Blank Runestones rolled at a high quality, and some Blood Vials. Each player can open it once with their own Bone Key.
Coffin
Gold Ingot 
Iron Sword Coffins always hold a stack of Arcane Essence and one or two low to middling Scrolls, along with Bones, Cobwebs and a bit of Gold. There’s a 15% chance of a 
Rotten Spell Book in each one, and they sometimes have an Iron or Diamond Sword with a spell already imbued into it, or a piece of enchanted Iron or Diamond armor.
Wall Container
Skeleton Skull 
Emerald Wall containers are smaller and a lot more common. They’re mostly junk like Rotten Flesh and Cobwebs, but they’ve got a decent chance of a Skeleton Skull and a small chance at a Scroll, a Blank Runestone or a ring, so it’s worth opening every one you pass. The Ancient Knowledge Fragment turns up in them now and then as well.
Decorative Pots
Bone 
Cobweb 
Pots are everywhere in the Catacombs and a third of them are empty, but they’re quick to break and some of them hold Emeralds, Arcane Essence, a Diamond or a Skeleton Skull. The rarest thing in them is the 
Dead King Lullaby music disc, which only drops from these pots, so it’s worth smashing every one you find.
FAQ
Craft a Wayward Compass from four Arcane Essence, four Iron Ingots and a Soul Lantern. It points to the nearest Catacombs as soon as you pick it up, so just follow it and dig down when you’re above it. With cheats on, you can use /locate structure irons_spellbooks:catacombs instead.
He drops a pile of Arcane Essence, three bottles of Ink that are Rare or better, and two more rolls that will each be a high level Scroll or a Blank Runestone. There’s a 50% chance of either The Blood Staff or the Necronomicon as well when a player gets the kill, and everyone in the fight is given a Bone Key.
His corpse only wakes up when a player right-clicks it, or hits it while standing within 8 blocks of it. You can explore and loot everything else in the Catacombs without ever triggering the fight, although you won’t be able to open the Bone Vault until someone’s beaten him and got a Bone Key.
After you beat him, the Dead King leaves a soul behind on the throne. Craft a Dead King Phylactery from eight Dead King Phylactery Shards and a Soul Lantern, then right-click the soul with it. Shards come from the Bone Vault and from the Magic Spawners, and drinking an Ominous Bottle first makes the second fight an Ominous one.
