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.
Deimos is located within the Ancient Quarry 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 "Deimos" as a small portrait around the right-middle of the screen, showing that it is affecting the game rules. Specifically, it can disguise all combat encounters on the map, thus making it dangerous picking random fights. The reward for slaying this Scourge is the Deimos Skull, an artifact that gives you an additional four physical defense, fifteen more points of max health, as well as immunity to the Burn, Freeze, and Stun status effects.