Yes such a thing does exist but its rarely used live by performers. There are vocal harmonizers (popular make is Eventide, I believe) that can create up to like 7 other voices to be layered along with your original "dry" vocal, and you can choose how loud each one is and also what key it is in (with the original vocal as a reference), and delay it etc. etc. etc. Opeth is one band that I think could make good use of one of these devices, with the right programming and a good sound engineer.
Ive seen Opeth 1 time and from what I remember the sound engineer didnt have too much sauce on his vocal, but he still sounded very good imo. Seeing them this Saturday I cant wait.