If Grunder is to ever break, like really break and loose his humanity, there is a potential to fall back into a pure demonic form. Nightmares of what this could look like haunts Russel. It's not impossible to go back into your humanity, but when you take that agreement, you run the risk of breaking.

What is the 'agreement'? It's usually upon your death, as you lie dieing, and you've committed horrible deeds and sins, the demon that was following you will make you an offer. It can be anything, but often with the hopes that you will be spared from hell or have some form of power within it. That's not entirely a lie, because you'll get powers, but you are not magically a demon lord. You are a human with a tainted soul, and now a poltergeist.

The demon that you fused with looses their personality, and what they were known from will enable your worse attributes. You'll feel like there's another person inside of you, making you behave like someone unfamiliar, but the reward is loosing all inhibitions and social and moral confinements. The powers you gain will be similar to what the demon's powers were, and you could absolutely bond with a pathetically weak demon because THEY wanted immortality within you. You won't know what you agreed to until you've breathed your last breath.

Often times, most poltergeists will retain their humanity, so long as as their human memories stay strong with them. It depends on their motivation for accepting the fusion and what type of demon they bonded with. What breaks them is the longevity of forever. Forever seeing every scenario play through earth's history over and over again. Letting the darkness consume you and you can control your actions and fate. You are not simply thoughtless, but can rationalize with more complex emotions than kill, consume, and destroy.

It's obviously a hard thing to do while we're a live, so it's always best to live as much as you can.