Ruyi Jingu Bang, Xiaolian's Artifact.

Originally used as a tool to measure the depths of the rivers and seas. 

What was once a tool stored inside the underwater palace of the Sea King after it was used, it only became a weapon after it was put into the hands of Sun Wukong.

It's a legendary weapon which weighs a whopping 7,960 kg, but can shrink or grow as its owner pleases. It's known for its high utility.