Martian Probe

A probe cannot really hurt you. It has no attack beyond bumping into you, and its entire job is to spot you and report back. The light on the front tells you which state it is in: green while it is still searching, red the moment it has locked onto you. Once it turns red it gains the ability to move through solid ground and runs for the top of the map. If it gets away, Martian Madness starts whether you were ready or not, and it does not even need to reach the top of the world, since it vanishes shortly before that and the invasion still fires.

Only one exists at a time, and a Lifeform Analyzer will call it out before you spot it yourself.

You get a narrow window and most of your usual tools are no help. Homing weapons ignore probes completely and minions will not choose to attack one, though a minion that hurts things by touching them can kill it by accident. That leaves whatever you aim yourself, and it needs the range to chase something that no longer cares about walls.

The good news is that hitting it does not alert it. Damage will not turn the light red. Only being scanned directly does that, so spotting one first gives you the full window. They can also sit above the top of the world where you cannot see them and scan you from up there, which is where most surprise invasions come from.

Spawns are weighted toward the outer thirds of the map, never near the center, and heavily toward the space layer. Anything that raises spawn rates generally will help, so a Battle Potion, a blood moon or an eclipse all give it more chances, but the Water Candle is the standout because it multiplies the probe's own chance well beyond the usual boost.

The trap that catches people is background walls. Standing in front of nearly any wall blocks probe spawns outright. Fence and cloud walls do not, and they happen to block wyverns too, so a small walled platform up in space with a candle burning turns a long wait into a short one. Harpies still arrive, so bring something to clear them. None of it works while a boss is alive or another event is running, since both shut probe spawns off completely. In remix and zenith worlds they can also appear down in the caverns and the glowing mushroom layer.

The spawn rate drops off sharply once you have cleared Martian Madness even once, and a probe that escapes gives you no bestiary credit at all, only the invasion. Filling the entry takes ten kills. If you want it, get those kills in before you deliberately let one get away. The candle and cloud wall setup still works afterward, it just takes longer.