Living Armor Mob

Suggested name: Sentinel

A large, haunted suit of armor that guards Jungle Temples, Desert Pyramids and Strongholds. It spawns in or near an area with valuable loot (such as treasure chambers and libraries), dormant and motionless, and awakens whenever a player comes near it. Alternatively, if this is considered too similar to how the Warden works, the Sentinel could instead patrol a set area. 

It carries a sword in one hand and a shield in the other (which can be disabled with the upcoming Cleaving enchantment). Unlike other mobs it takes a more careful approach in combat, slowly moving towards a player with its shield raised (unless the player flees, in which case the Sentinel gives chase). If the player successfully flees, the Sentinel returns to its original position and goes dormant again (or returns to patrolling its original area).

Being a ghost, it’s classified as an Undead just like Phantoms. I imagine it having a dark, featureless face, except for two glowing eyes. Similar to Elder Guardians and Piglin Brutes it would be a sort of “mini-boss”, spawning in upon world generation with no way of respawning. In terms of stats I think it should deal less damage than a Piglin Brute, but have more health (or just a ton of armor, which would make a lot of sense...)

As a bonus, perhaps its appearance could vary depending on which structure it generates in?