Appearance:

The Grim Guardian is a zombie with darker blue clothes (similar to black) and pale skin. His eyes shine and are similar to an enchanted zombie, with the difference of instead of having some purple tones, they are oranges and red.

Grim Guardian is the ancient variant of the Zombie encountered during Ancient Hunts with the minimum offering of the following runes: The ancient resides within a Creepy Crypt-themed arena and possesses, at minimum, the following enchantments: Echo.

More info: https://minecraft.fandom.com/wiki/Minecraft_Dungeons:Zombie