Crafting
To craft the Death Note, you must drop:
1x Book and Quill
1x Wither Skull

To use the Death Note, you write the name of the mob that you wish to die in the second page.

Selecting a Target
Writing the type of mob (e.g. zombie, pig, villager, etc.) will kill the nearest mob of that type.
Example: Writing "Cow" in the Death Note will kill the nearest cow.

Specifying Target
If the words "random", "nearest", "closest", "farthest", or "furthest" are written before the name of the mob, then the Death Note will target the mob in the way specified.
Example: Writing "Random cow" will kill a random cow.

Killing by Name
If the mob you wish to kill has a name, either by name tag or if they are a player, then you can simply write their name instead of their type. If there are multiple mobs with the specified name, the target can be further specified using the keywords explained above,
Example: Writing "Notch" will kill the player named Notch.

Specifying Cause of Death
After the name of the mob you wish to kill, you can also specify a cause of death. These are written almost exactly as how death messages appear. If the mob cannot die in the way specified, they will die of a heart attack.
Example: Writing "Nearest zombie was struck by lightning" will cause the nearest zombie to die from being struck by lightning.

Valid Death Causes:

• blew up
• burned to death
• death.fell.accident.water
• died
  
• drowned
• experienced kinetic energy
• fell from a high place
  
• froze to death
• hit the ground too hard
• starved to death
• suffocated in a wall
• tried to swim in lava
• was impaled on a stalagmite
• was killed by [​Intentional Game Design]
• was killed by magic
• was poked to death by a sweet berry bush
• was pricked to death
  
• was shot
• was skewered by a falling stalactite
• was squashed by a falling anvil
• was squashed by a falling block
  
• was struck by lightning
• was stung to death
• went off with a bang
• went up in flames
• withered away

I felt some of the causes were too long, so I shortened some:

• fell
• was impaled
• was squashed
• was skewered
• suffocated
• was poked to death