Phantom

"This manta-ray–like monster is a flying creature that spawns at high altitudes. It is attracted to insomnia, and will find players who haven't slept for many days. It will swoop down in groups and bite your flesh."

- Jens Bergensten

Phantoms are undead hostile mobs that attack players who have not slept in over three in-game days.

Spawning
Phantoms spawn in the Overworld at night, or any time during a thunderstorm above the player if they have not entered a bed for at least three in-game days. Phantoms will spawn more often the longer a player has gone without sleeping.

For phantoms to spawn, the player must be above sea level and not have a block above their head which blocks light in any way (for example leaves will prevent phantoms from spawning as they have a light opacity of 2, but glass will not, as its light opacity is 0).

If the conditions are met, a group of 1–6 phantoms will attempt to spawn 20–34 blocks above the player, and off to the side by up to a taxicab distance of 9 blocks, every 60–70 seconds. The chance of a successful spawn is $x−3⁄x$, where x is the number of nights since the player last entered a bed, which comes to a $1⁄4$ chance on day 4, a $2⁄5$ chance on day 5, a $3⁄6$ ($1⁄2$) chance on day 6, etc.

Phantoms are counted towards the hostile mob cap, but don't adhere to it when spawning. Their spawning mechanic is based on the location of players in the world rather than by chance, unlike regular mob spawning.

Drops
Phantoms drop when killed:
 * 0–1 s (or up to 4 with the Looting enchantment) when killed by a player or tamed wolf.
 * 5 s when killed by the player or a tamed wolf.

Behavior
When idle, phantoms fly around in a circle at a constant y-elevation, leaving a trail of gray smoke while doing so. They have a large search radius, and occasionally swoop down or up quickly to attack their victim. If stopped or hurt during this action, the phantom retreats back to its original elevation. When idle over water, the phantom sometimes swoops into the water and dies as it is not pushed back up.

Phantoms are able to move through water at the same speed as they do normally as they cannot drown.

A phantom's body disappears when under the Invisibility effect, but its eyes and smoke particles are still visible.

Like zombies and skeletons, phantoms burn in sunlight.

Phantoms attempt to stay 16 blocks away from cats.

Undead
Like other undead mobs, phantoms are harmed by the Instant Health effect, healed by the Instant Damage effect, immune to the Poison and Regeneration effects, ignored by the wither, and affected by the smite enchantment.

Data values
Phantoms have entity data associated with them that contain various properties of the mob.

Trivia

 * The phantom was added into the game as a result of winning the MINECON Earth mob Twitter poll as "Mob B".
 * Phantoms are currently the fastest non-boss mob in the game, traveling at speeds of up to 20.83 blocks per second.
 * At one point before the release of 18w10a, the phantom texture was made red, but it was decided that it appeared too much like a Nether mob.
 * The phantom was originally going to have a mouth, like the sketch shown during MINECON Earth, but it was eventually scrapped because "it looked so much like a muppet".
 * Even earlier designs can be seen on Minecraft's Instagram account.