Path of Exile

A corrupted zone has an icon that looks like a heart on the minimap. When you approach it it'll say "corrupted" and probably have a bunch of mods listed. Enter it and fight to the end, where you'll find a vaal vessel. Using it will unlock this.

I found my first in the Ship Graveyard, but I've seen many in other places. Not sure if their appearance is random though, I would assume so.

DarqStalker stated that their appearance is random but the places they can spawn are set, he posted a link that shows them all : https://pathofexile.gamepedia.com/Vaal_side_area