Wraith

Wraiths are a night-time surface spawn that begins with Hardmode. No event or item is needed, though a new moon raises their numbers sharply, so the darkest nights of the cycle are the ones where walking home turns into a fight.

The other source is deliberate. Smashing a demon or crimson altar coughs one or two out immediately, at any hour of the day, and that still happens in a Journey world with the spawn rate dialed all the way down. A statue produces them before Hardmode as well, but they arrive empty: nothing they carry becomes real until the world has turned.

They ignore walls. A sealed room does not keep one out and it will rise through your floor mid-sentence. The body is close to pure black, so in an unlit area the only thing you can track is a pair of red eyes drifting toward you, and the Hunter buff barely helps because the sprite is semi-transparent to start with. Contact also has a fair chance of slowing you, which lands at the exact moment you were planning to leave.

They look like they fly freely and they do not. A wraith hovers a short distance off the ground and no higher, unless there are solid foreground blocks for it to swim up through. That one limitation answers every wraith problem you will have. Build your surface base six or more tiles clear of the ground and none of them can reach it, no walls required. Platforms are not a ladder for them either, though a platform already at your height makes a perfectly good floor for one to hover on.

Water is the exception. Out over the ocean they can appear below you and to either side and drift across, so open sea is not the safe fallback it looks like.

Since they spend half the fight buried in terrain, weapons whose swing keeps going through blocks are worth switching to. Spears are the obvious pick, because the thrust holds its hitbox against something inside a hillside where an arrow would simply stop at the surface. Trap corridors are wasted effort here: they pass through pressure plates instead of standing on them, so nothing you wire up ever fires.

Waiting for nightfall is the slow way to collect the banner or the Fast Clock. Altar smashing is faster, you were probably going to do it anyway for the ore, and it saves building a Wraith Statue setup. If you would rather do it naturally, pick a new moon night and stand somewhere the height rule works in your favor.

An old collector's card claimed that anyone eaten by an Eater of Souls comes back as one of these. It cannot be the only recipe, since wraiths turn up in crimson worlds and purified ones where no eater has ever existed.