Calming Potion

It cuts how often enemies appear and how many can be on screen at once. Nothing about the enemies themselves changes, so it does nothing for you in a straight fight. This is for the times when the background spawns are the problem rather than whatever you actually came to do.

• Looting the Dungeon with a pile of keys, where the chests are the goal and the residents are noise.
• Mining a meteorite, which otherwise means fighting Meteor Heads the entire time you are down there.
• Queen Bee and the Wall of Flesh, since both live somewhere with heavy spawns that pile on during the fight.
• Building anywhere you keep getting interrupted.

It stacks with a Peace Candle and with the buff you get from standing near sunflowers, and all three together make an area close to silent. The big events ignore it entirely: the Pumpkin Moon, the Frost Moon and the Frost Legion will keep sending waves regardless. It also thins out critters, which matters if you were counting on catching them for bait or a fishing quest.