monsters that have something to do with souls?

[Red Bear]

My players are going to do a ritual to transfer a soul from a magical object to a body specifically prepared to receive it, I liked the idea that a during the ritual the soul out of the body could attract specific monsters that wanted to eat it or something similar or wanted to posses the body instead of the soul. Could think of some monster that would appropriately show up during this type of ritual?

the party is 4 player of 8th level, but the cr is a secondary problem

[JackPhoenix]

Pretty much any fiend would be interested in either soul or the body to be possesed, any incorporeal undead may not care about the soul, but would appreciate a new meatsuit.

Night hags, succubi/incubi and devourers stands out.

[Lupine]

Well, if the soul is in suffering, it could attack with will'o'whips. Specters too. Unfortunatly, those are rather low level, but if you're up for mass combat, you just over-run them with the buggers. would be a tough fight, if enough of them came, particulalrly if you buffed their stats a bit. Other than that, Pheonix would be correct that fiends would be a good bet.

Personally, I would run lower power fiends, and a lot of them to make it feel like they're under heavy assault while they do this, because in the fiends mind, it's easy pickings.