Blackwater.jar

Blackwater
it is already running at you. It reaches you, and then it is not there. What it leaves on the ground is a Bottle of Blackwater.
Pick it up and you get the achievement. They will do everything to find it.
That is not flavour text.
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The three
Porifera — tall, thin, sponge-grey. It does not walk so much as arrive. Its movement updates five times a second and it covers a lot of ground between those updates, so you will lose it and then it will be closer.
The Minewalker — humanoid, carries a pickaxe. It does not path around your base. It digs a corridor through it, straight at you, and you can hear where it is.
bw-data — a hole in the render, full of static. It shows up once every five minutes and gives itself forty seconds. It is the fast one. If you are somewhere it cannot reach, it places blocks under itself until it can.
None of the three can be killed. They ignore you entirely if you are not carrying the bottle. They are not there to fight you.
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What happens if they get it
The save stops opening.
Drink the bottle yourself and the same thing happens. You get one long sound, you get dropped to the menu, and that world refuses you from then on.
Your files are not deleted — the world is flagged, and the flag is what turns you away. Everything is still on disk if you want it back, but the mod will not let you in, and that is the point. Make a backup before you play if you care about the world.
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Config
`config/blackwater.cfg`
`heraldDelayTicks` / `heraldDelayJitter` — how long you get before the first one comes. Default is ten minutes plus up to five.
`bwDataIntervalSeconds` — default 300.
`stalkerIntervalSeconds` — how often a Porifera or Minewalker is sent. Default 90.
`corruptionEnabled` — set this to `false` while you are testing, unless you enjoy making new worlds.
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Notes
Minecraft 1.12.2, Forge 14.23.5.2847 or newer. No other dependencies.
Singleplayer is what it was built for. It runs on a server, but the corruption kicks everyone, so consider what you are doing first.
Works in an existing world. The timer starts the next time you load it.
Drop the jar in `mods/`. Do not read the source first.