A revenant is a hostile flying mob found only in hell worlds or Pandora events. They are tall, grey, ghostly figures that shoot deadly smoke-like projectiles at players and can only be damaged by energy-based weapons. See also Dire Revenant.
Revenants will sometimes spawn when a player mines a bloodstone tombstone. Other than that, they spawn only underground in open areas or against earth backdrop. On their own, they simply hover around making breathy ghost sounds and looking scary. Get close enough for them to notice you, however, and you'll find they're hostile towards any human still clinging to life in the apocalypse. Like brains and other flying mobs, if they catch sight of you they'll approach and attempt to kill you. They can easily get an upper hand in battle due to their immunity to most weaponry and their erratic ability to disregard mundane obstacles such as walls.
Revenants are vulnerable to only energy-based weaponry, and even then can take quite a thrashing before they fall. Due to their ability to sometimes move through solid objects, the more open the battlefield is, the better your chances are. Additionally, they have the weakness of limited upward range, so if you have enough steam and space to stay above them they can't hit you.
Revenants were originally coded as walking, rather than flying, mobs, and appeared as players with randomized appearance customizations and overlaid in transparent black.