1How to craft it
Work Bench
Used to craft
Demon Altar
what it does
It cancels the safety of a town. NPCs normally suppress enemy spawns for a wide area around your base, and lighting one of these switches that protection off inside a large rectangle. Anything that usually refuses to appear near housing, or behind safe background walls, starts showing up again.
how to use it
It is a spawn farming tool. Build your farm wherever you like, including directly on top of your base, put a candle in range, and you get full spawn rates without leaving your storage and crafting stations behind. Wyverns, Chaos Elementals and the other enemies with picky spawn conditions ignore those conditions while it is lit.
It works from the hotbar too. Just having it selected applies the same effect with no icon on screen, which is handy when you only want the pressure on while you are actively farming.
when to switch it off
Activate a placed candle to turn it off when you are done. The Skeleton Merchant will not spawn anywhere near an active one, so if you are waiting on him, that is the first thing to check. It also silences the town music nearby, which is a reliable audio cue that you left one burning somewhere.