Since the collapse of the Other Side, mediums can no longer contact the spirits of the dead, as the spirits have now moved on to other afterlife dimensions that are currently inaccessible to those in the living world.
Gaining mediumship is regarded as a consequence and an undesirable supernatural occurrence. Humans are able to become mediums from dying a non-supernatural death and then be resurrected by a witch, either through magical or medical means, giving them a connection to the dead. Examples of mediums who gained their abilities by this process are Jeremy Gilbert, who was resurrected by magic, or Matt Donovan, who was brought back to life with CPR.
The other way was to be bound to the Other Side as its Anchor, such as Amara or Bonnie Bennett, as the supernatural role allowed them to exist both on the Other Side and in the living world at the same time.
Powers and Abilities
Mediumship: Mediums possess the ability to perceive ghosts who are otherwise normally invisible. Ghosts can be summoned just by thinking of said individual although the medium needs to have some sort of connection to the ghost in question when they were alive. However, it has been demonstrated that being around someone connected to the ghost they seek can be enough to contact the spirit if the medium had no interaction with them in life. Those who serve the role as Anchor to the Other Side were able to not only communicate with the spirits of deceased supernaturals, but also physically interact with them as a result of their existence in both dimensions.
Mortality: Mediums who gain their powers from resurrection are still human and share many of the same weaknesses as non-supernatural beings (e.g. age, decapitation, disease, heart-failure, suffocation, etc.)
The doppelgängers and the mediums are the only supernatural beings who have only one special feature.
Doppelgänger - mystical blood that can be used to bind and fuel powerful spells.
Medium - mediumship, or the ability to communicate with ghosts on the Other Side.
When the Other Side was near destruction, one could become a medium by performing a séance. However, it is possible that only witches were able to perform the ritual, as Davina Claire, a powerful witch, was the only person shown to be able to communicate with spirits after performing a seánce.
However, Davina had also previously died and been resurrected by magic when she was sacrificed in the Harvest, so it is possible she actually gained her mediumship abilities as a result of this.