The notion of Bound originated from one of the Grandmasters, who felt it was too sad for Ancients to live without their other half. Rather than search for their mate, Bound were born as "Twin Souls" so they could live out their lives together.
Later during the formation of Mu, those who lost their Bound and had to form new pacts to survive were dubbed "the Fallen." This followed the belief that in becoming Fallen, one must betray their destined partner.
It is considered more honorable to die with your Bound than live a life without them.
Illness Related to Bindings
There are many types of Bindings and each one requires the partners to fulfill some quota or need. This trait of Bound makes it extremely difficult for them to be apart for an extended duration.
Each Bound partner has their own limit. Eventually, this illness can and will result in death. If one partner dies, the other will almost always find their way to their dead mate and die alongside them.
Residual Pain & Death
In the event that one of the partners dies first and the other does not immediately follow, the Bound pair will not be reborn together. This increases the chances of them becoming Stained and/or ill.
Furthermore, they will experience residuals of each other's pain, even if they are far apart.