It's just a compulsion - no, there are no undead I need to heal - I just hate that this one spell eludes me. The UESP only lists it as being available via purchase but you have to buy it from Ronthil, who is a vampire merchant. For a Dawnguard like myself, he's hostile - unless there was some moment where I could have bought it from him waaay back, like, when I returned Serana to Harkon in the very beginning (please tell me if this is possible (if you know)).
Thanks.