Rainbow Slime

It is the largest slime in the game that is not a boss, and it hits harder than either slime monarch with the health to survive a first exchange. Meeting one by accident in early Hardmode is genuinely alarming, because nothing else shaped like a slime does that kind of damage. If you have Royal Gel equipped it ignores you outright, which is the calmest way to collect from one if all you want is the drop.

Two conditions have to line up: hallowed blocks to stand on, and rain falling. Nothing summons it and it is not tied to any event, so the weather is the half you cannot control. It will also spawn on hallowed blocks sitting under water, which opens up more usable ground than people expect. On the oldest console and handheld versions the rule is inverted, and it wants non-hallowed blocks inside the Hallow instead.

Enemies never spawn on the same screen as you, so whatever surface you lay down has to sit off to the side of where you plan to wait. Platforms count as valid ground, which makes a stack of them a cheap way to multiply the spawn area without hauling blocks around. A Water Candle and a Battle Potion speed the whole thing up considerably.

If you are laying Pearlstone or Pink Ice above the surface for this, leave a gap between it and anything you care about, or you will kick off a hallow conversion you did not ask for. It is one of the rare enemies a Lifeform Analyzer announces, so you can farm with the readout up rather than staring at the screen through a rainstorm.

The reason to hunt one is Rainbow Brick, which cycles color the same way the slime does, so this is a building project rather than a progression step. Luck feeds into the spawn roll, so ladybugs, gnomes and a Luck Potion all help if you are settling in for a session. If you have no plans for the bricks, there is nothing here worth waiting on the weather for.