The Sludge is a hostile mob from the Deeper and Darker mod that bounces around the Blooming Caverns of the Otherside. It’s a sculk version of the Slime, so it hops toward you in the same way and splits into smaller copies of itself every time you kill a big one.
It matters a lot more than it looks, as the Sludge is the only place you’ll ever get 
Resonarium. You’ll need that for the 
Resonarium Plate, which upgrades Diamond gear into Resonarium armor and tools, so you’ll probably end up hunting quite a few of them.
Spawning
The Sludge only spawns in the Blooming Caverns, where it shows up alone or in pairs. It’s one of the most common mobs there, along with the Angler Fish in the water, while Sculk Snappers only turn up there every so often.
Unlike a normal Slime, it doesn’t need a Slime chunk or a swamp to spawn, so you’ll find them anywhere in the Blooming Caverns. The biome is in the Otherside, which you’ll reach through the Otherside Portal in an Ancient City.
Behavior
Sludges come in three sizes, just like Slimes, and each size is tougher and hits harder than the one below it. A big one will jump at you and deal 4 damage on Normal each time it lands a hit, and when it dies, it breaks apart into two to four smaller ones that keep coming after you.
| Size | Health | Damage | What to Know |
|---|---|---|---|
| Big | 16 | 4 | Splits into two to four Medium Sludges |
| Medium | 4 | 2 | Splits into two to four Small Sludges |
| Small | 1 | None | The only size that can drop Resonarium |
The smallest Sludges don’t deal any damage at all, so once a fight is down to the little ones, you can take your time picking them off. They’re the only size that drops anything, which is why it’s worth finishing every one of them rather than leaving them behind.
Fighting a Sludge
Since a big Sludge turns into as many as sixteen small ones by the end, fighting in an open space helps you keep track of them all. A sword with Sculk Smite on it will deal extra damage to every size, as the Sludge counts as a sculk mob.
It isn’t needed for the Sculk Slayer advancement, unlike most of the other sculk mobs in the mod. You’ll still want to fight plenty of them, though, as they’re the only way to get the Resonarium you’ll need for the upgrade.
Drops
Only the smallest Sludges drop anything, and even they don’t always drop it. A single big Sludge can still give you a few pieces once all of its small ones have been killed, so it’s worth clearing the whole group.
Sludge Drops (Small Only)
Each small Sludge drops up to one Resonarium, and Looting adds up to one more per level. A Looting III sword makes a big difference here, as it can turn a fight with one big Sludge into a good handful of Resonarium.
What Resonarium Is For
Four pieces of Resonarium and four Scutes from Turtles or Armadillos make a Resonarium Plate, which upgrades Diamond gear at a Smithing Table, along with a 
Resonarium Upgrade Smithing Template. You can use Resonarium to copy that template as well, so you’ll want plenty of it saved up before you start upgrading.
FAQ
Only the smallest Sludges drop anything, and they drop up to one Resonarium each, with Looting adding more. Big and medium Sludges split into smaller ones instead of dropping items, so you’ll need to finish off every small one.
Killing small Sludges in the Blooming Caverns is the only way to get Resonarium, as no other mob or chest has any. A big Sludge splits into several small ones, so each fight can give you a few pieces.
It only spawns in the Blooming Caverns, which is one of the biomes of the Otherside, alone or in pairs. It doesn’t need a Slime chunk, so you’ll find them all over the biome.
The Sludge works just like a Slime, coming in three sizes and splitting into smaller copies when it dies. It’s a sculk mob, though, so Sculk Smite deals extra damage to it, and it’s the only source of Resonarium.
