ignis fatuus

noun2

1. •
   A will o' the wisp.

   • Fiery spirits or devils are such as commonly work by blazing stars, fire-drakes, or ignes fatui; which lead men often in flumina aut praecipitia, saith Bodine[…] — 1624
2. •
   (figurative) A delusion, a false hope.

   • […] they think this is written on purpose to be published; and that this defence is so published, that it may serve as an Ignis fatuus to mislead them […] — 1660