In the underworld timeline, the influx of souls allowed many soul based technology to be created. One of such technologies is Soul Link. When a small number of researchers studied the crystalline souls found by explorers, they found that similar souls are able to interact with each other if one is living and the other is crystalized. They were able to make a rune to make the process a lot more reliable.
Soul Links require a compatible living soul and crystal soul to be linked via a Soul Link rune. The rune is fairly easy to make. It takes a rune mage at about five seasons to make a Soul Link rune. Additionally, there is no specific rune chisel needed to make a Soul Link, but the closer that the rune is to the body of the living soul's body, the stronger the connection.
Crystalline souls that are similar to the soul of a living being can be used to give the living being the knowledge and some abilities of the crystalline soul. Additionally, the living being can commune with the other soul while asleep. This Soul Link is kept until another the living being initiates another. A Soul Link can be used with other runes to force vampirism out of vampire spawn and dhampires, but it cannot heal true vampires or vampire lords.
When Soul Links were first used, one of the researchers described how he felt a similar feeling before. Not only does having a Soul Link give a user extreme Deja-vu, but it also led to the discovery of a natural Soul Link that is created when a soul is fractured and placed into multiple beings. These "fractured souls" are the natural kind of Soul Links. A soul can be "allergic" to Soul Links and will not join a Soul Link or will become a crystalline soul if in a Soul Link.
A crystalline soul compatible with the target, a Soul Link rune and 1 hour is required to activate a Soul Link. The target has +1 to all rolls they are proficient with and, when they use an ability or spell slot of level 5 or lower, the target may roll a d20. On a 20, the target regains the used ability or spell slot. While nonmagically asleep, the target may try to commune with the soul it is linked to. Only one Soul Link may be active on a target.