How To Get Ruby Fire Mystery Eggs In Merge Dragons?
Harvest from Dragon Trees – You will get a lot of Dragon Tree leaves and rarely Ruby Fire Mystery Eggs
Mythical Idols – You can merge and tap the Idols for good chances to get this egg
Possibly from the Ruby Fire Mystery Nest seen below
Dragon Egg Chest 23 Gems harvested from Dragon Trees
Dragon Nest Vault 115 Gems harvested from Dragon Trees