Science of Ignis Fatui:

The Latin plural for a will-o’-the-wisp is ignes fatui. Meaning and Etymology

Literal Translation: The singular term is ignis fatuus, which translates directly from Medieval Latin as “foolish fire” or “giddy flame”.

The Breakdown: Ignis means “fire,” and fatuus means “foolish,” “fatuous,” or “silly”. It earned this name due to its erratic, flickering movement and its habit of making fools out of travelers.

Grammar: To make it plural, both parts of the compound noun change to their plural forms: ignis becomes ignes (fires), and fatuus becomes fatui (foolish ones). Dual Meanings

The term ignes fatui carries two distinct definitions depending on the context: 2 days ‘til ‘Will-O’-The-Wisp’ – Facebook