The Great Pen is a key item from the platformer/RPG video game Paper Mario: Pen Power, which is the 5th title in the Paper Mario series. It allows the user to paperize the game world and draw things such as barriers, spikey balls and bridges. It uses ink taken from the enviroment to work.
The Great Pen has many uses. The first is the ability to priefly paperize the game world, making it two-dimensional. When in this mode, Mario and Luigi can use it to draw many things. Doing this uses up ink, which must be refilled at a literal Ink Well or any other place with ink.
Many years ago, the ancients of the Mushroom Kingdom found the pen along with many other things. They soon found, however, that it was very similiar to their own pens, but could actually create items, platforms and wheel in the world. Deciding it's power was too great, they locked it up underground. Later on, Magic Mike found it and hosted a big magic show with it as it's main focus. However, a mysterious figure attacked with it's airship, finding the pen and shooting ink everywhere. After recovering, Mario and Toadster found the pen and used it to uphold the figure. In the end, Mario throws it into the lava, destroying it.