Ghosts are aggressive mobs that can appear with a 10% chance when digging up a Grave with the Shovel or Regal Shovel, or when all the graves are dug in Maxwell's Graveyard. They make unique whispering noises when attacked or defeated. They do not attack like regular mobs, instead inflicting damage whenever next to the player or mob. This damage comes in the form of small attacks that are dealt very quickly over time. Ghosts don't drop anything upon "death".
In the Shipwrecked DLC, Pirate Ghosts replace Ghosts. Aside from looking like a Pirate, Pirate Ghosts are identical to Ghosts. Pirate Ghosts can spawn when a Wreck is hammered or a Watery Grave is fished. Additionally they will spawn from unfished Watery Graves during a Full Moon.
In the Hamlet DLC, Ancient Spirits replace Ghosts. Aside from looking like a Pig, Ancient Spirits are identical to Ghosts. An Ancient Spirit will spawn when the player mines certain statues or when looting inside of the Ancient Pig Ruins, particularly when stealing items from pedestals in a hidden room. During the Aporkalypse, every room in the Ruins will contain multiple hostile Ancient Spirits.
Ghosts are fairly slow, and the player can easily outrun them. They have a very large insanity aura, so they can be noticed pursuing the player via the aura while they're not visible on screen. Ghosts will attack anything in their path, often killing small animals in a single blow, so they can be led to mobs if the player wants to lose them.
A Ghost will have a sad appearance, similar to Abigail, when at low health.
Below is the number of hits it takes with each weapon to kill Ghosts when playing with characters with a default damage modifier. The Weather Pain is not included due to the random nature of its projectile.
Because Ghosts will disappear after a while and do not drop anything, it is best to just let them be. The easiest way to kill a Ghost is by luring it to a few Pigs or a Beefalo; this way the Pigs or Beefalo will kill the Ghost.
A Ghost can also be used as a grinder; let it kill whatever enemy it comes across, then take the drops.
Ghosts are very slow and can be easily outrun. After the player has run a distance, the pursuing Ghost will likely have found another mob to be aggressive against.
Despite levitating above the ground, Ghosts are not immune to Tooth Traps.