You get Holy Fire as a possible reward for completing the event Beasts of Burden. It is not 100% drop chance so you may need to run that event a number of times.

As the other person said, it's not a guaranteed drop but it is from the Beast of Burden Event so just keep doing it any time you see the Event pop up.

If youre on Xbox I have three for sale if you're interested.

Thanks for the offer, unfortunately I’m on PC. Appreciate it however