This is one of the several Scourge enemies in the game. On the timeline at the top of the screen, a timeline of turns will show you the progression of time, whether it will be day or night soon, as well as more critical indicators like Chaos or Scourge activations. As a precaution, every land biome often has at least one Scourge; certain scenarios may allow objectives such as Cult Devices or Realm Dungeons to appear once per biome as well.
Disciple is located within the Temple of Darkness tile. Your best hint is to scour the first few biomes you start in and see if you can find its dwelling early. If activated, you will see "The Disciple" as a small portrait around the right-middle of the screen, showing that it is affecting the game rules. Specifically, it will make it permanently raining, thus often messing with some of your movement rolls. The reward for slaying this Scourge is the Disciple Hood, an artifact that offers 8 evasion, +1 find range on the world map, an additional Focus slot, and spreads Shock Immunity to the party.