Writhan

Writhan spawns on swamp ground between 2,000 and 8,000 m from the centre of the world, and nowhere else (the inner edge was 5,000 m until patch 1.0.15 pulled it in). It is the only creature in the game with a distance rule, which is why the swamp next to a first base never has one and the swamp on the far side of the map does. Every other line of its spawn entry is ordinary: day or night, no boss kill needed, one alive at a time.

The world spawner runs per 64 m zone. Each loaded zone keeps its own timer for each creature entry, and every 4,000 seconds of world time (about 1 h 7 min) it makes one attempt: a 10% roll, then the checks. A zone that has never been loaded has no timer yet, so it makes its attempt the moment you walk in. That is the whole trick to finding one: fresh swamp zones inside the ring are the only ground where the odds are on your side. Standing still in swamp you have already walked gives each zone one more 10% roll every 1 h 7 min of world time, and a night's sleep skips only a fraction of that.

With the timer passed, the game picks a point 40 to 80 m from a player and asks whether it qualifies: swamp biome, not a swamp edge (all four corners of that zone must be swamp), ground between 2 m below and 10 m above sea level, slope under 35 degrees, 2,000 to 8,000 m from the centre, no player within 40 m, not inside a workbench or ward radius. It tries twenty points before giving up. And if a Writhan already exists anywhere in the loaded area around that zone, nothing spawns at all.

Put together: crossing ten never-loaded swamp zones inside the ring gives about a 65% chance of one appearing, twenty-five zones (the whole loaded area around you) about 93%. Once it is dead the next fresh zone can produce another. Stars roll at the default 10% per star, so one in ten is a one-star and one in a hundred a two-star, and a two-star drops four times the roots.

To see the ring in your own world, open the seed on the seed map and turn on the "Writhan zone" terrain layer: it draws the 2 and 8 km rings and fills the swamp between them, bright where the spawner will accept a point and dim along the edges it refuses. The seed finder has the same measure as a filter, "swamp Writhan can spawn in", for picking a world before you commit to it.

Across 99,986 generated worlds the median holds 5 km² of qualifying swamp, about 1221 zones; one in ten holds 5.75 km² or more and the best seen 7 km². No world had none, so every seed has somewhere a Writhan can appear; the difference is how much walking it takes.

Undead, weak to slash, resistant to blunt and fire, immune to poison. An axe or a sword, not a sledge, and not a torch. At speed 2 it cannot close on you, and its bite is only 40 pierce and 30 poison, so this is a safe farm rather than a fight. The part that matters is what happens when it dies: the death spawns an explosion, 60 blunt, 50 chop, 50 pickaxe and 40 poison across 6 m, dodgeable but not blockable, and it hits props as well as characters. Chop and pickaxe are the two damage types armour never reduces, so a hundred of that lands on anything wooden within 6 m in full. Land the last hit in the open, step back, and never finish one next to a portal or a fence. Every kill gives 1 to 2 Writhan Roots, doubled per star, which build the Shield of Roots (1, then 2 more per upgrade), the Lox Fur Hood (2, then 1 more per upgrade) and the Birds' Nest (2). The first kill also sets a key on the world that puts the Crown of Roots in the Bog Witch's stock for 3,000 coins.

Each 64 m zone keeps its own timer per entry and rolls the chance once per interval while loaded; a zone never loaded before rolls the moment you arrive. The spawn lands 40 to 80 m from a player, never inside a base unless noted, and "max near you" counts the loaded area around that zone. Dungeon and camp spawners are separate and not counted here.