This item can drop from the Gul'dan encounter in the Nighthold on Mythic mode. It can also be listed on the Black Market Auction House.
Here you can buy Fiendish Hellfire for EU or US Servers.
You'll get guide for getting Fiendish Hellfire
You'll be able to get Fiendish Hellfire
Additional budget for Character transfer or Character boost to max level. BMAH Services in Shadowlands now requirement to have a max level character on server, so Trial character will not work.
Free time on evening, if you choose selfplay option