Sand Poacher

The underground desert is backed by wall from end to end, and this thing walks on wall. That combination is the whole problem: no ceiling or side counts as safe, and the moment it is on a wall with a clear view of you it speeds up hard. Running away after it has spotted you usually fails, which is a rough surprise for someone who has just stepped into Hardmode.

Its touch applies Acid Venom, which drains you steadily and lasts longer the harder the world, so one brush in a tight tunnel can be what kills you rather than the hit itself. Minions and weapons that pass through blocks are the early answer: let something else fight inside the wall while you keep a tile of air between you. A pocket you dug yourself beats a natural passage.

They only appear in clean or Hallowed underground desert. If Corruption or Crimson has taken your desert, the poachers stop and you get that biome's roster instead, worth remembering when you are picking a spot for a desert farm or hunting the Fried Egg.