Minecraft is a survival game, but developer Mojang is apparently not above making it into a survival horror game. The profoundly spooky and very murderous Wardens are horrors of the upcoming Deep Dark underground biome, eyeless creatures that seem to want to keep the deathly quiet of the underground pristine.
Attracted to vibrations and the shrieks of their Sculk brethren, Wardens come burrowing out of the ground to attack basically anything. It is horrific, honestly: They come barreling out and just murder whatever they find. Wardens can find you not just from vibrations in Sculk, but from scent as well: They'll sniff around to find you. God help you if you collide with them.
The most sure-fire way to summon a warden is to step on the spreading fungal goop that is Sculk, letting the vibrations reach the bony hands of a nearby Sculk Shrieker. Trigger enough shriekers and a Warden comes. As the Warden arrives, it inflicts a new status called darkness that makes the world ever-darker to you: Which means just what you think, more monsters.
They're gruesomely powerful and dangerous, as befits a survival boss you avoid rather than fight—which they are. Indeed, they have the highest health and damage of any naturally-spawned mob in the game... and they disable any shields they hit. (Golems, eat your heart out.)
The latest Minecraft snapshot, the Mojang equivalent of a beta test, looks like it's going to roll the Warden into the base game. You can read more about that on the Minecraft website.