This is a consumable in Dark Souls 3.
Practical Use:
Consume the Soul to gain 400 Souls.
Lore:
Someone once owned this soul, or better yet, this soul owned a body, long forgotten and deserted.
The Unkindled is a vessel for souls, and as such, you can give this Deserted Soul a home once again, absorbing it, making it a source of strenght.