People get possessed all the time.

Harmon knows how this goes. He's from Nevarra, and he's read books. Plural. If you want to fight a dragon you need to find a Kindly Mentor who can teach you all their Secret Strategies. So all he needs to do is find this old Tal-Vashoth dragonslayer his cousin keeps going on about, get him to teach Harmon about dragons, convince everyone else to go along with his dragon-hunting plans, fight a dragon, and win. Easy, right? Starring one cheerful qunari grandpa, one grumpy necromancer grandpa, and a bunch of Nevarran mercenaries who are about to have a very educational experience.