What if you could try to capture player souls? the FOV could go down as the player is being captured, alerting them. Also, it would have to be nearly impossible to successfully capture a player. When successfully captured, the soul type matters. If the player is mortal or has not yet decided their soul type, the player's screen will turn black until the soul vessel shatters after a couple seconds, allowing the player soul to move into their body again. If the player has the demon soul type, either you could never capture the player or the soul vessel would simply pull the soul out of the body, but not capture it.

Final Idea for ease of reading:
On the very slight chance it works, the player's screen turns black, before releasing their soul at the capturing player's location. Mortal souls can only possess their own body. It might give requiem players even more attrition than if capturing an animal failed.

Ah, so you could try capturing a player's soul but it would never succeed ? Could be interesting, but I'm afraid it would make players think there is a trick or an upgrade to make the process work... As for the "pulling the soul out of the body" part, I believe that would be redundant with the Penance effect.

I also realized another issue with capturing a player soul: the fact a soul vessel can be thrown into lava or a cactus. Maybe the soul vessel could release the soul if the vessel is destroyed? Also, it could be that either way, it is possible to capture a player soul for a couple seconds before the vessel breaks. The only difference is the appearance of the soul, as well as the mortal soul only being able to possess its own body.

Ah, that was your suggestion, I initially thought you suggested the screen would turn black during the animation, then the vessel would just break. I don't believe actually putting the player's soul in the item to be a good idea, notably for the reasons you mentioned.

That could work. The player's screen could just turn black during the animation, not linking it to the item.