Resonarium is a material from the Deeper and Darker mod that drops from the smallest Sludges in the Blooming Caverns of the Otherside. It’s what you’ll use to make 
Resonarium Plates, which upgrade Diamond armor and tools into Resonarium gear.
The armor is the reason to collect it, as each piece blocks a quarter of every hit you take. You’ll also need it to copy the 
Resonarium Upgrade Smithing Template and to repair the gear, so it’s worth gathering more than the plates alone call for.
Obtaining
Sludges work like Slimes, so when you kill a big one it splits into smaller ones, and those split again until they’re as small as they get. Only the smallest Sludges drop 
Resonarium, and each one gives you 0 or 1, with Looting adding up to one more per level.
A single big Sludge can split into a lot of small ones, which makes them a decent source once you’ve found a few. Sludges only spawn in the Blooming Caverns, a cave biome of the Otherside, so that’s where you’ll do all of your farming.
With a Looting III sword you’ll get up to 4 Resonarium from each small Sludge instead of 1, which speeds things up a lot. The smallest Sludges can’t hurt you at all, the same as the smallest Slimes, so it’s only the bigger ones you’ll need to watch out for.
Uses
Resonarium Plate
Most of your Resonarium will go into Resonarium Plates, which take 4 Resonarium and 4 Scutes in any shape on a Crafting Table. The Scutes can be 
Armadillo Scutes or 
Turtle Scutes, so brushing a few Armadillos is the quickest way to get them.
Each plate upgrades one Diamond armor piece or tool at a Smithing Table, along with the template. The four armor pieces take 4 plates, which is 16 Resonarium, and the five tools take 5 more, so a full set of everything needs 36 Resonarium.
Since the armor and tools draw on the same plates, it’s worth deciding which you want first. The Resonarium Armor is the part with an effect, as it blocks damage, whereas the Resonarium Tools are only a small step past Diamond.
Resonarium Upgrade Smithing Template
The template is what lets a plate upgrade Diamond gear, and it’s used up each time. It has a 45% chance of being in an Ancient City chest, 32% in a Stronghold corridor chest and 28% in the common reward from a Trial Chambers Vault, and the top chests of an Ancient Temple can have one too.
2You’re able to copy it by crafting it with 7 
Diamonds and 1 Resonarium, which gives you 2 templates back. The template and the copying recipe were added in 1.4, so they aren’t in the 1.20.1 version of the mod, which stops at 1.3.3 and upgrades Iron gear without a template.
Repairs
Resonarium is the repair material for all Resonarium gear, and each one restores a quarter of the item’s durability at an 
Anvil. The armor can’t take Mending, so you’ll be using Resonarium to fix it for as long as you wear it.
The armor wears down every time you’re hit instead of taking damage yourself, so it’ll need repairs a lot more often than normal armor. It’s worth keeping a stack of Resonarium in a chest at home for this, rather than using it all on plates.
FAQ
Kill the smallest Sludges in the Blooming Caverns of the Otherside, as each one drops 0 or 1 Resonarium. Big Sludges split into smaller ones when they die, and Looting adds up to one more per level, so Looting III gets you up to 4 from each small Sludge.
It’s crafted into Resonarium Plates with 4 Scutes, which upgrade Diamond armor and tools into Resonarium gear at a Smithing Table. It also copies the Resonarium Upgrade Smithing Template with 7 Diamonds, and it’s the material you’ll repair Resonarium gear with.
Ancient City chests have a 45% chance of holding one, Stronghold corridor chests 32% and the common Trial Chambers Vault reward 28%. The top chests of an Ancient Temple in the Otherside can have one as well, and you’re able to copy it with 7 Diamonds and 1 Resonarium.
They don’t, as only the smallest size drops it, so you’ll need to keep killing the Sludges that split off until there are none left. A big Sludge splits into several medium ones, and each of those splits into several small ones, which all have a chance to drop it.
