This complex ritual ties the physical and spiritual energies of two target creatures together. It requires the intense work of 5-10 spellcasters combining their energy, though the task becomes easier if one or both target creatures together. The ritual takes approximately a week (160 +/- [1d20] hours) to perform correctly and requires spellcasters to work in shifts to maintain concentration until the spell is complete. For each target creature, there must be one spellcaster who knows them personally. The closer the connection, the higher the chances of success.
Once the ritual is successfully completed, the two targets will share any damage or conditions inflicted upon either, including death. Over time, their personalities, desires, and values may be influenced by one another as well, though usually one personality tends to be the more dominant. This effect is especially noticeable if the targets have both consented to the binding. The ritual is presumed to be permanent, though bound pairs have rarely been observed beyond a handful of decades. It is possible that the magic effects will fade with time, or that the bond could be broken in some other way.
If the spell fails, there is a chance that the two target creatures or any of the spellcasters participating in the ritual will suffer a permanent loss of function, either in body, mind or soul. The worst failure may even kill any or all targets or participants. (Work with your DM to determine how this manifests in your own game.)
* - (One from each target creature: a small vial of blood; a sample of flesh, muscle, and bone tissue (a small body part, such as the end of a finger or tail, can serve as all three), a portion of soul in a container, an object of great significance (an example might something specially created by the target, an object they treasured, or a written copy of a story or piece of knowledge which was somehow important to them))