Hunter Potion

Everything hostile lights up red through solid rock, and critters and town NPCs you have not found yet light up green. The glow only exists on your screen, so it illuminates nothing and helps you not at all in a fight. What it removes is the surprise from a cave you have never been in.

When you are hunting something specific. Mimics and Nymphs are obvious with it and easy to walk straight past without it, and the same goes for Truffle Worms and the rarer critters. Captive townsfolk waiting to be freed, the Goblin Tinkerer and Tavernkeep among them, plus wanderers such as the Skeleton Merchant, all show up green through walls, which turns a long sweep into a short one. A Lifeform Analyzer pairs well, since it tells you something rare is nearby and you can spend a potion in response rather than drinking them on a schedule.

• Burrowing enemies stay visible while they are inside blocks, so worms and Man Eaters stop ambushing you.
• Sealed pockets underground give themselves away through whatever spawned inside them, which is a fast way to find unexplored space.
• The cultist fight becomes readable, since the copies do not light up the same way the real one does.
• Wraiths ignore it completely and stay as hard to see as ever.

Every one of them needs a Shark Fin, which is the actual bottleneck. If you find yourself drinking them constantly, a Shark Statue on a timer solves the supply problem permanently.