I made this Fjarnskaggl farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick this Herb from skill 1, you will get the same number of herbs as if you had Herbalism 100.
One of the new features in World of Warcraft Legion is that herbs have ranks now. You can get the new ranks from quest items that you get by picking [Fjarnskaggl]. Once you finish the quest you get from the herb and the follow up quest, you will get the new rank.
Rank 1 : Gather Fjarnskaggl more effectively.
Rank 2 : You can get [Blood of Sargeras] when gathering Fjarnskaggl.
Rank 3 : Mount quickly after gathering any herb in the Broken Isles.
If you have the [Sky Golem] mount, you can use that instead of the Demonsteel Stirrups, and if your class has a tanking spec, you should switch to that while farming, so you won't get dazed and you don't have to use Stonehide Leather Barding.
Don't forget to enchant your gloves with [Enchant Gloves - Legion Herbalism]! This enchant brings gathering time down from 1.5 seconds to 0.5 seconds. This is really important if you gather while mounted, because mobs can still interrupt your casting.
Every other herb in the Broken Isle have a chance to spawn a creature that gives you more herbs, but it seems Fjarnskaggl doesn't spawn anything.
This is the best farming route, but if you barely find any herb it means someone is already farming there, then you should try the alternative route below.
You will pass a small horde camp while you follow this route, so if you have an alliance character, just stick to the right side of the hill and you won't get attacked by the guards.
This route is not bad, but it can be really hard to farm here if you don't have Sky Golem mount or don't use Demonsteel Stirrups because there are a lot of parts where you can't avoid aggroing a bunch of mobs.
If you tried both routes and you still barely find herbs, it means both routes are heavily farmed, so you should try to come back another time.