what slime rain is
Slime Rain turns the sky into a spawner. Slimes drop onto the surface across the whole world, and every one of them is hostile, including the low tier ones that would normally ignore you at full health. More of them fall through the background layer behind the terrain, which is how you can spot the event coming from a distance. A run lasts between 9 and 15 minutes in singleplayer and a flat 15 minutes in multiplayer. Logging out does not pause the timer, and a boss being alive does not stop it from starting.
the kill counter
Underneath the weather effect this is a kill quest. Reach 150 slime kills and King Slime drops in on top of you. If you have beaten him in that world already, the bar is 75. Only slimes that share the common blue slime identity in the game code count, so the ordinary colored slimes plus Pinky, Baby Slime and Jungle Slime move the counter while Ice Slimes, Corrupt Slimes and other biome specials do nothing for it. Standing in a biome with its own slime type also means those variants keep spawning at their usual rate rather than raining.
Nothing cares how the slime died. Lava, a dart trap, a Slime Statue wired to a one second timer, a town NPC taking a swing at something: it all counts. That is also why the event can trigger when you did not want it. Kill nothing at all and it expires on its own.
If King Slime spawns and then leaves without dying, which is what happens if you exit the world while he is up, the counter resets and the next one costs 225 kills, or 102 if he had already died before the event began. Killing him ends the event on the spot, and that is the only way the achievement attached to it unlocks.
when it can start on its own
A natural Slime Rain only rolls in the morning, between 4:30 AM and noon, and only once somebody in the world is past 140 maximum life and 8 defense. Expert worlds can roll it before that gate at a much lower rate. At full odds it lands on roughly 12 percent of days. Hardmode cuts that, and so does having killed King Slime already, which is why the event feels common in a young world and rare in a finished one.
- Any invasion, moon event, Blood Moon, Solar Eclipse or Lantern Night in progress blocks it.
- Rain blocks it, for manual summons as well as natural rolls.
- A brand new world sits out its first 24 to 48 minutes. Skyblock worlds are exempt.
- Singleplayer adds a cooldown of 84 minutes to 3 hours after each event. Multiplayer has none, so two can run back to back.
- Remix and the Mole people seed never get it at all.
If you would rather not wait on the dice, throw a Slime Crown into Shimmer. That starts one immediately and ignores every condition above.
an arena that actually helps
Build two flat layers with a gap between them. Slimes land on the top deck and line up where one wide swing clears them, and when King Slime arrives you drop to the lower layer and fight him without a crowd of small bodies soaking your hits. A sealed box with a single tile gap is fine for the farming phase, since arrows, bullets and yoyos all pass through it, but get out before the boss lands. He teleports, and he will teleport into the box with you.
A Water Candle and a Battle Potion push the spawn rate up when you want the counter moving faster. Going the other way, Sunflowers, a Peace Candle and a Calming Potion thin the crowd when the event catches you mid build. The Journey Mode spawn rate slider does nothing here, since the rain ignores it.
what is worth farming
In an early world this is the best Gel source in the game by a wide margin, and Pinky turns up often enough that Pink Gel stops being a rarity. The Slime Staff is the real target, and the sheer volume of kills makes the event the least painful place to chase a drop that thin. Banners for the common slime types fill in along the way, and King Slime himself pays out gear that matters most on a first world, when nothing you own is good yet.