You have decided to become a Tempest rogue, and most likely already have two Essence Containment Apparatuses and can’t find the third. You’ll find a group of demons in the Storm Coast, near the Solitude Camp, on top of a hill. They are not part of a rift (see below).
If you are not sure how to get there, you can watch this video I recorded. The demons are no longer on top of that hill because I killed them, but they will be there. The Darkspawn wasn’t actually there the first time and I tried to avoid him, unsuccessfully, but the demons will be on top of the hill.