They roam the deepest dungeons, or watch over commonly used areas from hard-to-reach places. Often, they drop some of the richest loot in all of Vana'diel. Players spend months and years hunting them. Real-money traders often camp them literally non-stop.
They are Hyper Notorious Monsters, often referred to by adventurers as "HNMs."
For some, HNM camping is a one-time affair in hopes of getting rich, or obtaining a rare/exclusive item. For others, HNM camping becomes a way of life in Vana'diel.