Revenant

Revenants are an undead mob, which would look like a smaller wither, but white and with only one head. They would be armed with an iron sword. They would only generate naturally in mausoleums. They would drop 0-2 bones and a Revenant Heart when they died, with a chance to drop their sword. However, what is unique about them is that they can move through solid blocks to reach their target.

Revenants can 'see' any mob in a 40-block radius, and will attack any non-undead mob within a 16-block radius. However, revenants cannot 'see' through gold blocks, so any mob behind such a block, even if the block is encased in other blocks, cannot be seen by the revenant. If a player or non-undead mob is seen, then the revenant will move straight towards their target, passing through all blocks in its way. If it loses sight of its target it will return to the nearest 1x2x1 space; as such, revenants will not loiter in walls.

Revenant Hearts can be used in brewing. When added to a potion of night vision, they make it a potion of spectral vision. Any mob under the effects of such a potion will be able to 'see' mobs within a 40-block radius, except if there is a gold block in the way. To the player, any mobs would be outlined in a light blue.

Revenants can also be summoned by the player, by placing two bone blocks one on top of the other, then placing a skeleton skull on top, similar to the construction of a snow golem. Player-summoned revenants will not attack the player who summoned them, but other than that will behave normally, attacking any non-undead mobs, and even other players, who get too close.