The Spectral Explorer

Add a new ghost mob, the Spectral Explorer, that spawns in and around existing structures (Desert Temples, Ocean Monuments, Mansions, Ancient Cities, Strongholds, etc.). These represent previous adventurers who fell while trying to reach or conquer these places. Immune to ranged attacks (arrows/tridents/snowballs), fire/burn damage, and explosions, players are forced to fight them in close-quarters melee combat.