The Sculk Leech is a tiny hostile mob from the Deeper and Darker mod that comes out of fake 
Ancient Vases, 
Infested Sculk and a few other hidden spots in Ancient Cities and the Otherside. It only has 4 health and bites for a single point of damage, but every bite also drains some of your experience, which is what makes it worth dealing with quickly.
You won’t find Leeches wandering around on their own, as they only appear when something releases them. Most of the time, they turn up when you break a vase or mine the wrong block, or during a fight with the Stalker, which can pull a whole swarm of them out of the ground.
Spawning
Sculk Leeches don’t spawn naturally in any biome, so the only way to meet one is to set one off. There are a few different things that can release them, and it’s worth knowing all of them, as most of them look like ordinary blocks until you break them.
- Fake Ancient Vases in Ancient Cities and Ancient Temples, which release one to three Leeches when they don’t release a Stalker
- Infested Sculk mined without Silk Touch, which lets one Leech out
Crystallized Amber with a fossil inside, as about 1 in 5 of these hold a Leech instead of an item- The Stalker, which pulls one to three Leeches out of the ground every couple of seconds during its ring attack
Breaking any of these blocks with a Silk Touch tool stops a Leech from coming out at all, so it’s a good idea to carry one when you’re exploring an Ancient City. Silk Touch keeps a Shriek Worm from coming out of Infested Sculk as well, since the block will just drop as normal Sculk.
Behavior
A Sculk Leech will go straight for the nearest player as soon as it’s released, and each bite deals 1 damage on Normal. That on its own isn’t much, but every bite also takes 4 points of experience away from you, so a few Leeches chewing on you can cost you a fair bit of the experience you’ve saved up.
Dealing With a Swarm
Since they only have 4 health, a single hit from any sword or axe will kill a Leech, so it’s more a case of finding them than fighting them. They’re slow and very small, so it’s easy to lose track of one in the dark, which is why it’s best to clear them up straight away rather than leaving them behind you.
During a Stalker fight, Leeches will keep coming for as long as its ring attack lasts, so stepping back out of the ring’s 10 block reach is the best way to stop more of them from spawning. You can then pick off the ones that follow you before going back in to fight the Stalker.
Drops
The Sculk Leech has one possible drop, but it doesn’t always give it, so you’ll need to kill a few to collect any amount. It’s still worth picking up, as it’s one of the items you’ll need for some of the better gear in the mod.
Sculk Leech Drops
Each Leech drops up to one 
Soul Dust, with Looting adding up to one more per level. Soul Dust is used to make 
Soundproof Glass and the 
Soul Elytra, and it can turn a Potion of Invisibility into a Potion of Sculk Affinity at a Brewing Stand.
Sculk Slayer
The Sculk Leech is one of the mobs you’ll need to kill for the Sculk Slayer advancement, so make sure to finish one off the first time you see it. Weapons with Sculk Smite deal extra damage to it too, though that won’t matter much with only 4 health to get through.
FAQ
One hit from any weapon is enough to kill a Leech, as they only have 4 health. To stop them appearing in the first place, break Ancient Vases, Infested Sculk and Crystallized Amber with a Silk Touch tool, which keeps anything from coming out.
Every time a Sculk Leech bites you, it takes 4 points of experience away along with the 1 damage. It isn’t much from one bite, but a group of them can drain a fair amount, so it’s worth killing them quickly.
They come out of fake Ancient Vases, Infested Sculk mined without Silk Touch and some Crystallized Amber, and the Stalker summons them during its ring attack. They never spawn in any biome on their own.
They drop up to one Soul Dust, and Looting adds up to one more per level. Soul Dust is used for Soundproof Glass, the Soul Elytra and Potions of Sculk Affinity, so it’s worth picking up.
