Instability

A third side effect of Horcrux creation is that the Master Soul itself becomes unstable (even with creating just one Horcrux). The precise dangers of this spiritual destabilization are not currently detailed explicitly throughout the franchise, however, some can be gleaned from the events in the books.

For example, the creation of Voldemort's seventh "Horcrux"—Harry Potter—is known to be the direct result of this.^[2] When Voldemort was hit by the back-fired Killing Curse at Harry's parent's home in Godric's Hollow, it caused Voldemort's soul to split, with one fragment remaining in him and the other displaced part immediately seeking out the only other living thing in the room and latching onto it—Harry Potter. However, this parasitic fragment of Voldemort's soul that attached to Harry is not actually a Horcrux, since it was not created intentionally and the necessary parts of the Horcrux creation process were not carried out.

Limbo

The final known side-effect of Horcrux creation is the inability to move on from Limbo after death. This is seen when Voldemort's Killing Curse (after the destruction of all the other Horcruxes) rebounded and finally ended his life once and for all, his broken and mangled soul^[6] was forced to exist in the stunted form of a flayed and mutilated baby that Harry saw in King's Cross during his visit to Limbo, unable to return to the land of the living, unable to become a ghost, and unable to go to the land of the dead because his soul was maimed and unwhole. It is unknown if this was a standard fate meted out for all Horcrux creators, or if it was unique to Voldemort due to the number of his Horcruxes. Regardless, reconciliation cannot occur after death, as the soul's state at death remains forever, so the greatest of all consequences incurred by Horcrux creation may be the possibility of eternal limbo of the soul.