Slime Staff

The Slime Staff is the rarest weapon in the game, with a drop rate of 0.01% (1/10000) from most Slimes and from the Slimed Zombie. It summons a baby slime that fights for you until cancelled, or used again. It can be obtained in pre-hardmode. The slime spawned by the Slime Staff cannot be killed.

A way to obtain the Slime Staff without much effort is to construct a statue farm with one or more Slime Statues, leave it alone for awhile, and occasionally check if the staff has been dropped yet.

Trivia

 * Resembles a Dango from the anime Clannad.
 * The staff itself might be a Shillelagh.