Cburr has given you some really good ideas to get started. Once you have a draft from those ideas, and don't worry about word count at that point, be sure to include some figurative language like a simile, where you say that something is "like" something else. Each idea you have put into the draft needs at least one specific detail to help the reader see the pen and feel what it is feeling.
Also, remember that when you personify an object like the pen, you are giving it human characteristics. This is a technique called anthropomorphism. Write from the point of view of the pen as though it is alive.
After all this drafting, then use your Word Count function in your word processor. You'll need to judiciously cut some description in order to meet 50 words!
I would talk about the pen's reaction to what the person holding it is writing.
You can also include:
the sound of the pen on the paper
what the paper feels like to the pen
whether the person is squeezing it or holding it gently
whether their hand is sweaty, cold, etc.
how it feels to have its ink (lifeblood?) drained away to make a good/bad/whatever message
These are just a few ideas.
The pen personified may feel like a servant destined to oblige his master, namely the writer. The pen feels the tight grip of the writer and the writer using him whenever he likes. In a long session of writing, especially at odd hours of night, the pen feels tired and sleepy. Whatever fame the writer earns is to the writer's credit. Nobody remembers the pen.