Variants
404040Giant Worm spawns as 3 variants that share one bestiary entry; each row is one variant's exact stats.
Drops
Banner
Every 50th kill of this enemy drops Worm Banner.where you run into one
Giant Worms live in the rock under the surface and travel through solid ground rather than over it, so the first warning is audio. The burrowing noise speeds up and gets louder as the worm closes on you, and a faint trail of particles marks the ground it has already chewed through. A sudden jump in that sound usually means it is about to break out of the wall beside you. Newer players get caught because the worm reaches a little higher than expected, turning up while you are still sinking your first shaft rather than deep in the caves.
what has to be true before one appears
These follow the stricter spawn rules, so a lot of situations quietly block them. A town NPC living nearby shuts them off. Player-placed safe walls behind your character shut them off too, which is why a papered-over mineshaft or a walled base stays clear. The game also checks that you have done something in the last few seconds: moved under your own power, jumped, or used an item. Riding a conveyor belt while idle does not satisfy it. Holding or standing near a Shadow Candle throws every one of those checks out, which is the quickest way to make them come to you. They also refuse to appear while you are in an Ice biome, and in the Cavern the Underground Hallow blocks them as well.
killing one
All the segments feed one shared health pool, so it does not matter where you connect. That favors anything landing several hits per swing. A Spear is the standard early answer, since a single thrust rakes across the body as it passes you. The other half of the problem is simply seeing it. Setting the worm alight with a Flaming Arrow or splattering it with the Slime Gun leaves a marker on the segments so you can read where it is heading underground and step off the line before it surfaces.
after the world turns
Hardmode gradually retires them in favor of the Digger. In the Underground layer the two share a roll and the Digger wins most of the time, so Giant Worms get scarce. In the Cavern the replacement is total and they stop appearing there at all. If you want the Worm Banner, finish that farm before you break the altars.