Quintus

Quintus was the fifth son of the King of Stormhold.

Book
In the book he was killed by eating eels poisoned by Septimus.

Movie
Prior to the events of the film, Quintus was murdered by his youngest brother Septimus, who struck him in the head with an axe while he was asleep. Septimus may have incriminated Quartus in some way, as a deleted scene from the film has Quintus blaming Quartus for his death, only for Quartus to correct him - something it is implied he has done repeatedly.

As a ghost, Quintus still retains the axe in his head. He also appears to be the least intelligent out of all his brothers; repeatedly blaming Quartus for his demise when Septimus was responsible, using a fictional word - "rample" - in a game of "I Spy" played between him and his brothers in a deleted scene, and even cheering when Septimus's corpse was attacking Tristan Thorne. Whether or not this is the result of the axe in his head remains unknown.

Once Tristan Thorne became the next King of Stormhold, Quintus passed on to the afterlife with his brothers.