Battle Potion

Doubling the spawn rate only helps when you genuinely want more enemies in your face. That means farming a specific drop, or forcing something rare to show itself at all: Martian Probes over the surface, Truffle Worms in a mushroom biome, Prismatic Lacewings in the Hallow after dark. It also thickens up a Blood Moon or a Solar Eclipse, which is the fast route to a Money Trough or a Broken Hero Sword.

It raises how often the wandering and bound townsfolk appear as well, so if you are combing the caverns for the Goblin Tinkerer, the Mechanic, or the Wizard, drinking one shortens the search considerably. The Skeleton Merchant turns up more often under it too.

A Water Candle stacks on top of it, and the two together are the standard farming pair. Pulling the other way, a Calming Potion still ticking from earlier eats a large slice of the increase, and so do the Sunflowers planted around your base, which is why a farm built at home underperforms one built out in the wild. The Pumpkin and Frost Moons set their own rates and override all of it, so drinking one before a moon event is money burned.

Spawns are generated per player, so everyone on the server drinking at once produces far more than one person's doubled rate. That is excellent for grinding an invasion and genuinely dangerous everywhere else, since the cap on how many can be on screen goes up with it.

The Hardmode Dungeon under this buff is unmanageable in almost any gear, because the residents there do not get weaker for arriving in numbers. Never drink one before a boss either. A fight you had comfortably handled becomes a corpse run the moment ambient spawns start piling in behind the boss. Unlike a Water Candle, this one comes off with a right click on the buff icon, which is worth remembering while things are going wrong rather than afterward.