The stand arrow is a powerful item introduced early in Diamond is Unbreakable.

In Part 4, it becomes a major plot driving force for its role as the source of many new stand users; and it is revealed that its influence extends back to Stardust Crusaders.

50,000 years ago, a meteorite crashed around the underpopulated region of Cape York, Greenland. In 1978 a mineral excavation team uncovered an unknown virus within the rocks. It had been laying dormant, but had the poisonous potential of human extinction.
Certain people had a quality that allowed them to survive this virus and develop a superpower. Several hundred years before the discovery of the virus, a man who wanted the power of god took the rocks and crafted them into arrows. These arrows had been all too lost until 1986, when an Egyptiann ruins investigation team, including a young "Diavolo" uncovered six of them from underneath the Egyptian desert. Eventually, these were stolen by Diavolo who sold five of them to "Enya the Hag". The arrows allowed the wielder to unlock the psychic power known as a "Stand" to those who had the potential. The six arrows were eventually distributed between various people, some of them being "Keicho Nijimura brother of Okuyasu Ninjimura", "Yoshihiro Kira, father of Yoshikage Kira" and
"Jean Pierre Polnareff, the badass frenchman we all miss."

The arrows effect also travels through family bloodlines, if someone in your family possesses a stand, you will also possess a stand.
"Johnathon Joestar"'s body, which was possessed by DIO, was pierced with the arrow, awakening the stands in the rest of the family, with certain people lacking spiritual strength suffering from strain ("Holy Kujoh, mother of Jotaro Kujoh" and "Josuke Higashikata, son of Joseph Joestar") A family member without the spiritual strength of a stand will suffer from a severe fever.

When a stand user is pierced with a stand, their stand will gain a new ability, as it happened with Yoshikage Kira's Killer Queen. However, in the only known example of this, the arrow has to move by itself and pierce the user willingly.

If an arrow hits the stand instead of the user, the stand evolves and it's powers become drastically stronger than usual. This form is called a Requiem Stand, the only instances it was mentioned is Gold Experience Requiem and Silver Chariot Requiem, the original Requiem. Two of the arrows are still missing in the world...

#JOJO