#3. The Fiend is a lost soul and he's been waiting for Alexa Bliss to become powerful enough to bring him back
The Fiend was essentially killed by Randy Orton back at WWE TLC. Now, the former Universal Champion has to find a way to link his soul and his body back together.
Let's be honest, despite all of the science and research that has gone into finding out what happens when we die, there isn't anyone alive who can answer that question.
It's thought that when we die, our soul leaves our body. It is then able to go to heaven or hell but if the person in question is accepted in neither place, they are left to wander The Underworld.
Perhaps The Fiend has been wandering The Underworld over the past few months hoping that Alexa Bliss becomes powerful enough to bring his soul and body back together. Dark magic can be so strong that despite what is left of The Fiend's body, he would still be able to re-inhabit it if Bliss is able to cast the spell.