Lifeforce Potion

The gain comes in blocks tied to each full hundred points of your maximum, so a bar sitting just under a round number gets nothing for the leftover. If you are a crystal or two short of the next hundred, closing that gap first is worth more than it looks, because the potion works off the rounded figure and not your exact total.

The extra bar is the point, and the quieter benefit is regeneration, which scales with your maximum and creeps up alongside it. In expert and master fights where you are already stacking everything you own, this is one of the biggest single survivability gains available for one slot. Drink it during the buff round before you summon, not once the fight has started.

Prismite only comes out of hallowed water. Fish for it on the surface rather than underground: very few other special catches compete for the roll up there, so the same number of casts returns far more of them. That fish is the entire bottleneck on this potion.

Deep gold chests near the bottom of the world and shadow chests both stock these, so it is possible to walk into a pre hardmode boss already buffed. Most people never check, because the crafting route does not open until much later.