Warden

A warden is a mob summoned by sculk shriekers. They are completely blind, and thus rely only on vibrations and sense of smell to identify the player and other mobs. Nonetheless, they are extremely powerful and one of the most dangerous mobs in the game.

Spawning
If a sculk shrieker is enabled too many times, a warden will spawn by digging out of the ground. If a warden hasn't been noticed or gotten angry at anything for 60 seconds it will dig back underground and despawn.

Behavior
Wardens are completely blind, if a vibration is made near them, they will go to the place from which the vibration came from. They are also able to smell players and mobs that are too close to them.

The core on its chest creates a low throbbing or "heartbeat" sound as it pulsates in tandem with the Darkness effect. The sound and animation speed up rapidly as the warden gets more suspicious of or closer to attacking a player or mob.

Warden will prefer to track down mobs that it is more suspicious of, rather than the closest one.

Once Warden is certain about nearby player or mob, it will roar at it, and proceed to attack.

Warden will also react to vibrations produced by entites such as projectiles or minecarts, but never attacks them.

The warden is classified as an undead mob, which means it takes extra damage from the Smite enchantment, gets healed by Instant Damage status effect, damaged by Healing status effect, and immune to Poison and Regeneration status effects.

Trivia

 * Killing a warden doesn't count for the Monster Hunter advancement, nor is it needed to receive the Monsters Hunted advancement.
 * Previous names considered for the warden were the "stalker" and the "hollowed".
 * The stalker iteration was tall, asymmetrical and had a glowing core. It occurred in totems throughout the Deep Dark. The stalker would stand completely still to trick players into taking its core and aggravating it, and could transform into a one block high form to prevent players from digging away. That iteration, specifically its appearance and transformation mechanic, felt too clanky and was more goofy than scary, which is why it was scrapped.
 * The hollowed was designed with the intent of inciting trypophobia and had white flesh with holes in it. That iteration was deemed too gory by the developers and had to be toned down.
 * In the sixth episode of “The Secrets of Minecraft” it was jokingly revealed that the warden’s personal name is William, similar to how the ender dragon, the wither, and player characters all have personal joke names. The word was also jokingly broken down into the words: war and den.
 * Kingbdogz stated in a tweet that the souls in the warden’s chest is incredibly important to its lore.
 * Kingbdogz also stated that the warden is based on the "monster" heard at the end of "11".

Concept artwork
Some concept art in this gallery was originally made for the Caves & Cliffs update.