My gut check says CR 4, but I could be convinced to 5.
A dark green, nearly black, gelatinous mass.
Flareaters are intelligent and highly dangerous relatives of green slime...
I think we're done with the Flareater, which of the remaining Oozes would you like to do next?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.