Soo...for starters, the quest item that you need to get that quest started is kinda rare ( as i've heard) and you get it by gathering starlight roses in Suramar.
It is important to note that you are able to get it only if you have already done the previous part ( find a blade while gathering roses in suramar and fought with Tharillon around Moonguard's Stronghold).
Once you get this quest started, head to Azsuna co-ordinates /way azsuna 55.55 25.38 where you will find yourself infront of a stream/river. If you look closely to the rocks ( as the previous guys above are showing in their videos) you shall notice a hole on the ground.
Picture of the entrance : http://imgur.com/a/Qf1vn
Once you enter the cave you'll see pack of leaves on the floor, right click on them. Dialog window will pop up, you use your 20 starlight rosedust ( that you've collected by gathering starlight roses) and Tharillon appears.
http://imgur.com/a/Tq15r
Dude is a 10m NPC, not really dangerous especially if you do it as a tank. Remember to interrupt his "Drain Energy" ability or stun him.
Drain Energy : Channeling spell that once completed benefits Tharillon with a buff that increases his haste and damage done.
Also he is casting a second spell that unfortunately i don't remember the name that is giving him 4% haste and 1% maximum health or something like that.
Between the 2, the second is insignifficant as you can easily solo him with item level around 801 as tank if you just use your cooldowns correctly and interrupt Drain Energy.
Once killed, loot his corpse and off you go back to Dalaran to learn your rank 3 gathering starlight rose !
Whooooooooop ! Wait a second, there's also a treasure in that cave pick that shiny first and then be on your merry way.
Hope i helped, happy hunting !