Warden as seen in Minecraft Live 2020
This article is about the new hostile mob coming to 1.17 Cave update.
Wardens are the first blind mobs that live in caves and can "sense vibrations". They track down the player by "Vibrations" made by the players whenever they move. They cannot (Presumably) detect crouched movement.
On a full bar of (presumably Netherite) armor, the Warden can knock out around 6 hearts of health, which possibly makes it the strongest mob against a full set of armor.
Warden concept art from Minecraft Live 2020.