Bone Serpent

The Bone Serpent is the reason a first trip to the underworld tends to end badly. It is long, quick and hits harder than anything else you have faced before the wall, and it does all of that while ignoring the terrain you were using for cover. The worst version of the encounter is inside a ruined house, where the walls block your view and your escape but do nothing to stop the serpent.

It also does not stay where it spawned. Once one has locked onto you it will follow you up out of the underworld, so climbing back toward the cavern layer does not shake it off.

Most enemies that pass through blocks refuse to spawn while you are standing in front of proper background wall, which is the usual trick for making an underground base quiet. The Bone Serpent breaks that rule and spawns anyway. If you are building a hellevator landing or a bridge across the underworld, walling the area does not buy you safety, and you should plan on one arriving mid build.

Every segment feeds one shared health pool, so damage multiplies with each part of the body you can touch at once. Aiming at the head is the slowest way to do this.

• The Vilethorn plants a piercing line inside the body and hits repeatedly as the serpent slides through it.
• Flails swing back and forth across several segments per throw.
• The Crimson Rod cloud rains on everything underneath it, which suits a target that spends most of its time strung out horizontally.

Single target weapons will finish the job, but not before it has passed through you two or three times, and at that stage in a run that is usually fatal.

It is one of the very few enemies from before the wall that gets no stat upgrade when the world turns to Hardmode in expert or master. Almost everything else down in the underworld scales up around it, so the serpent that terrified you on your first visit gradually becomes the least dangerous thing in the room while its neighbors get worse.