What is called a serious piece of writing whose ending always sorrowful?

A serious piece of writing whose ending is always sorrowful is called a tragedy.

Here's why:

* Seriousness: Tragedies deal with weighty themes like human suffering, loss, morality, and the flaws of humanity.

* Sorrowful Ending: The protagonist, often a noble character, faces a downfall, often due to their own actions or flaws, and experiences a tragic end.

Here are some examples of tragedies:

* Literature: Shakespeare's "Hamlet", Sophocles' "Oedipus Rex", Arthur Miller's "Death of a Salesman"

* Film: "Citizen Kane", "Casablanca", "Schindler's List"

* Opera: "Tosca", "La Bohème", "Carmen"