soul corrupting item ideas

[JungleChicken]

I have a group through Curse of Strahd and i am trying to add a hook that fits each character as a draw or lure from the dark powers.

I have a good idea for 3 of my 4 players but the 4th is a WArforged that is trying to prove if he has a soul, what a soul is and things of that nature.

I am trying to think of a good item or power that could be granted that could be used to help achieve that goal in some way but be corrupting, even slightly at first. The idea is that if the character indulges enough to be corrupted then the dark powers just might not want them to ever leave but it'll be a road of their own doing

[HappyDaze]

Docent. Evil but subtle.

[Keltest]

An empty Lich-style phylactery. They can try and use it to see if it accepts their soul, at which point... it enters the phylactery and becomes subject to its influence. Assuming you want them to actually have a soul.