This string is used by many witch coven quests as well as this vampire quest.
Since dfall treats daedra princes as factions, and I think == means faction name, the usage here is appropriately ==daedra_. So it should be a name like Hermaeus Mora or Namira.
_daedra_ is used by QBN to define enemy types like daedroths and frost daedras. I don't see it used in a QRC and I'm not sure what it would do if you tried.
I submitted a PR to fix this issue, as well a a similar error in quest R0C60Y24 which uses the ==vampire_ quest macro to get the name of the vampire clan.