The main purpose and power of a horcrux was to anchor the creator to the mortal realm for the sake of achieving immortality. As long as at least one horcrux existed, the creator's soul would be bound to the world of the living. Hence, if the body was destroyed, the soul would remain as a lingering spirit instead of passing through to the other side. Though being in such a state of existence was what very few would prefer over death, the soul could possess others to regain physical form, which shortened the host's lifespan drastically. It was also possible (through a second party) to use certain potions to create a rudimentary body or even the true body of the horcrux creator's soul to possess. Because of this purpose, a horcrux was usually kept hidden and dormant, and its other powers were rarely explored or utilised.

The fragment of a person's soul within a Horcrux was capable of thinking for itself and had certain magical abilities, including the ability to influence those in their vicinity and affect them mentally.

However, when away from a Horcrux its influence and effects would begin to wane and then disappear and those influenced would return to their normal selves.

A person with an affinity for the Dark Arts and having a malicious disposition, on the other hand, would be strengthened by the influence of a horcrux.

If a person was more emotionally vulnerable and was in physical contact with it for long enough, it was possible for the soul inside the horcrux to take control of them.

If given enough time and opportunity, the horcrux could drain enough of the victim's life-force and soul to manifest itself into a more physical form, at the cost of killing the victim should it continue to completion. This quasi-sentient entity that was capable of sapping life-force to create a physical form differed from a mere memory, which the diary's manifestation claimed itself to be, as no mere charmed object could achieve such a feat.

In this way, a horcrux could gradually feed on another person's life and negative emotions to strengthen itself and increase the ability of the soul fragment within to act independently in the physical world.

Horcruxes also possessed some last line of defence against destruction. The fragment of soul within the horcrux seems to be able to sense impending threats and can act to defend itself.