Terraria has an unimaginable variety of potions. Out of those, the buff potions provide some kind of literal buff to the players. The Battle Potion is one such buff potion.
Using the Battle Potion will double the spawning rate of enemies and critters. Usually, players use the Battle Potion to summon rare bosses or to get some precious drops.
Terraria: Getting and Using the Battle Potion
Crafting the Battle Potion is easy. You need 1 Bottled Water, 1 Deathweed, and 1 Rotten Chunk. The Rotten Chunk can be replaced by Vertebra.
Now, to brew the potion, you can use the Placed Bottle or the Alchemy Table. If you want to save ingredients, then go for the Alchemy Table.
The buff lasts for seven minutes and it has a very fast use time. If things get out of hand, you can always turn the buff off. When used together with Calming Potion, the effect of the Battle Potion will be reduced to 54%.
Typically, players use the Battle Potion to summon rare bosses. It also sets the spawn rate for events like Solar Eclipse and Blood (they do not have a pre-determined spawn rate).
Sometimes the Battle Potion also increases the spawn rates of NPCs like Skeleton Merchant, Goblin Tinkerer, and more.
Lastly, if you are feeling exceptionally brave, you can use the potion while in Dungeon or Hardmode Dungeon. However, note that things can get out of hand and give rise to some nasty situations.