Quintus was the fifth son of the King of Stormhold.
In the book he was killed by eating eels poisoned by Septimus.
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.
- Quintus is the shortest of the three princes dead at the beginning of the film - himself, Quartus and Sextus. If we assume that all the brothers would have grown up to be of similar height had none of them died, then this could indicate that he was the first of the brothers to die.
- Another hint that Quintus died first is the fact that he believed Quartus had killed him; if Quartus had died first, this wouldn't have been possible, and if Sextus had died first, then Sextus' ghost would likely have witnessed Quintus' own death and thus told him what had happened.