Cursed Skull

It drifts toward you through solid rock, which makes the dungeon's tidy stone rooms no defense at all. Close up it stops advancing and circles you for a moment, and that circle is the tell. What follows is a sudden straight lunge, the same wind and commit pattern the Eye of Cthulhu uses. It is slow enough to read easily, but it hits far harder than anything else floating around a pre-Hardmode dungeon, so reading it is the whole job.

Contact has a real chance of leaving you cursed, and cursed locks out items entirely: no weapon, no healing potion, no grappling hook out of trouble. A few seconds of that in a corridor with skeletons already closing is usually the whole death. Plan to survive the follow up rather than heal through it, because healing is precisely the thing being taken away.

Brick walled stretches of the dungeon produce them at a much higher rate than the rest of it. They do appear elsewhere in there, just at a fraction of the frequency. If you are hunting rather than passing through, find a bricked corridor and stay in it instead of wandering.

This is the most reliable source of the Nazar, the accessory that blocks the exact debuff it inflicts, and the Ankh Shield needs one. Do the grind before Hardmode. Once the dungeon fills with new enemy types after Plantera, the spawn pool thins out and the same hunt takes several times as long for the same result.

Because they come through walls, you can hit them through walls. A spear or the Vilethorn kills one from the far side of a block with no risk at all. The cost is drops landing inside the stone where you cannot reach them, which means either mining in after each kill or wearing a Treasure Magnet and letting it pull them out for you.