[ What did the human soul get out of it? That was an interesting question, although becoming a shinki was not something to aspire to. This information is fortunately, applicable to all of the gods so it's not above his paygrade at all. ]

That's right. It's the soul of someone who's died, usually in an unexpected way.[ A tragic, unexpected way. ]

Souls like that are vulnerable. They died before their time, or unexpectedly which means the dark one wasn't there to escort them to the afterlife. They remain there to be preyed upon by the ayakashi. A spirit like that can be taken in by a god. If they aren't they may be devoured by the ayakashi,or they may let go of what binds them to the living world and move on. Some are reincarnated and others just cease to be. What they get out of the deal is a purpose and protection.

[ Yato considers his thoughts again, just for a brief moment. ] They sacrifice their lives in return and are tasked with serving the god who took them in. There's nuance, it's not as permanant as it sounds.

[ Possession? By what? ] If you mean the ayakashi, ending their posession is part of my job, yes. The human would be unharmed from severing that tie.