The Relic is a piece of cyberware that the entire main story of Cyberpunk 2077 revolves around.
Relic is an interface and component of Arasaka's experimental, flagship project publicly known as "Secure Your Soul." It uses the concept of personality transferal by converting the data in the human nervous system into a digitized engram and stored as a construct on the Relic biochip. The purpose of "Secure Your Soul" is the ability - currently only available to the wealthiest clients - to communicate with deceased loved ones. Their personality constructs, which are preserved on the Relic biochip mean that in practice, death is no longer the ultimate barrier. However, it doesn't take a genius to realize that the potential for the Relic technology is much higher - Arasaka currently is not sharing any information on its further plans for the program.
The Relic is a piece of prototype bio-cyberware that Saburo Arasaka commissioned Anders Hellman to create. It was intended to allow him to transfer himself into another body, to allow him to live on forever by constantly swapping bodies.
V is commissioned to steal this piece of bioware from the Arasaka corporation, but it had already been stolen by Yorinobu Arasaka, who was keeping it in his hotel room. The chip has to be kept at a cool temperature to ensure that it is not damaged. Exposure to temperates above the recommended levels will slowly destroy the chip, as shown in the heist mission, where Jackie Welles has to slot the chip into himself to stop it being destroyed by the heat.
V and the Relic
As Jackie is dying, he gives V the Relic to ensure it is not further damaged. V slots the biochip in and seems unaffected, but after Dexter DeShawn betrays them and shoots V in the head, the chip is damaged.
It starts to write over V's personality, replacing it with that of Johnny Silverhand; V experiences a constantly deteriorating state of health, caused by the Relic. Johnny constantly appears to V, talking with them throughout the game and even helping them to stay alive. When V is killed by Placide, Johnny not only brings V to a motel nearby, but also revives them.