The Spade's Blade

In answer to your question, you need a Jeweled Spade Handle drop to start this chain. Keep picking Starlight Roses until you get one.

The NPC you need to kill (Herbalist Tharillon) is located up on a cliff, in an area with many level 110 Nightfallen mobs. You will need to kill him and then loot a chest nearby to complete this quest. (Note: technically, it's just the chest you need; if you can get the chest without killing him first, that would work too; I did it by killing him, personally. Edit: I am not sure if it is still possible to loot the chest without killing him; judging by posts on this page it seems the opinions about this are mixed).

Here is an image showing him and the area around him, and a zoomed out image to give a better perspective of his location here.

If you would like to see screenshots showing how to reach his location on the cliff beginning at 26, 27 in Suramar, I have put up several images showing this here, including directions and coordinates for each image. (Note: this post was originally created before flying was possible in Legion, and shows how to get to Tharillon without flying).

If you'd rather skip the screenshots, then start at the coordinates indicated above and continue up the path toward the buildings; keep heading up the stairs higher and higher. Note that there will be many level 110 Nightfallen mobs along the way; eventually you will reach an area where you can head right and then down a short flight of stairs, into a grassy yard. Cross the yard to get to a building, which will have a large back exit. Go through the exit and to the right, and you will reach the NPC you need.

For the part with the yard and the building, there were so many mobs that I used an invisibility potion to bypass them and ran to the area in which I took the screenshot at the top of this post (this one). Alternatively you could die near the NPC and then res at the area where I am (~34.4, 22.7 in Suramar). As very helpfully noted by Beriberi above, if you pull Tharillon into the place where I'm standing on the cliff, most likely only one other mob will pull along with him and you won't be overwhelmed by mobs.

Please note that I was level 110 when I did this; I read on here that, for some people, even when they pulled him from the place shown above, a lot of level 110 mobs still aggroed instead of just one additional one. I don't know for sure, but I wonder if this may be level-related; that is, maybe if you're lower level then due to your larger aggro radius you might pull more mobs even from the 'hidden' area. (Yes, there are people who have noted in the comments here that they've done this quest at levels below 110). It was not a particularly challenging fight for me at level 110, while I read on here that it can be quite a hassle if you're relatively low level.

To clarify for people trying to start this quest:

You must FIRST pick Starlight Roses within Suramar until you recieve the "Jeweled Spade Handle". Take this item back to the herbalism trainer in Dalaran then you will recieve a follow up quest to go kill Herbalist Therallion in the Moon Guard Stronghold.

Hope this helps!