In the OED, it means ignis fatuus, will-o’-the-wisp.  Does it mean something different here?  I believe so!

• 08.117 where we last saw the elf-fires.’
• 08.124 where elf-fires had been.

“elf, n.1.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/60431. Accessed 13 September 2017.