Digger

The Digger is the Hardmode version of the burrowing worm you spent all of pre-Hardmode ignoring, and it lives in the same underground and cavern rock. It does not fully push out the Giant Worm on current versions, which still surfaces occasionally after the wall falls. The two share a kill counter and a banner, so any worm kills you racked up early carry over, and a Worm Banner you already have hanging works on Diggers without a single extra kill.

Diggers have unusually tight spawn conditions, which is why a walled base can go hours without seeing one and an open tunnel is full of them. None will appear if a town NPC lives nearby, or if the tile at your center has safe background wall behind it, or if you have been completely idle for the last few seconds.

Idle is stricter than it sounds. It means no movement input, no jump, no auto select, and no item use apart from a kite. Being carried along by a conveyor belt does not register as moving, so an AFK farm built on belts will never produce one.

A Shadow Candle throws all of that out. Hold one, or stand within range of a placed one, and every restriction lifts. That is the reliable way to farm them rather than waiting on luck.

Depth and biome gate it further. It will not spawn at underground depth while you are standing in an ice biome, and it will not spawn at cavern depth while you are in the underground hallow.

It burrows through solid rock and its whole body feeds one health pool, so the job is hitting as many segments as possible per attack rather than aiming at the head. Spears are the standard answer because the thrust reaches through walls and lingers long enough for several segments to slide across it. Damage over time works the same way: apply something like cursed inferno to a few segments at once and the ticks pile up quickly.

Beyond that it is not a fight to respect much. The real danger is getting pinned in a narrow tunnel while it passes through you repeatedly with nowhere to step aside.