This guide will help you farm the legendary weapon:
Rhok' Delar, Longbow of the Ancient Keepers
First of all, we will port to the Stairs of Destiny.
[Might and Magic] > Mission Area > The stair of Destiny.
Find the NPC called "Rhok' delar".
Accept his quest so we know what materials we have to farm in order to upgrade our bow.
The next step will be to go to the Vanilla Raid "Molten Core".
[Might and Magic] > Raid Instance > Vanilla Raid > Molten Core.
We will now have to find the Boss called "Majordomo Executus".
he will reward us with an item called "Ancient Petrified Leaf".
Accept the quest and go back to the Mission Area by using your [Might and Magic].
Visit the NPC Rhok' delar again and finish the quest from the quest item.
He will offer you the next quest in order to acquire "Ancient Rune Etched Stave".
To finish this quest we will have to kill 4 bosses and loot their heads.
NOTE: All 4 bosses listed now will drop [Heart of the Fiend] - make sure to memorize their Spawnpoints to farm this material.
Let us work down the list now by killing Simone the Seductress first.
[Might and Magic] > Worldmap > Azeroth > Kalimdor > Un'Goro Crater
Make sure to loot the Head and also [Heart of the Fiend] which will be needed for further upgrades.
We can also farm material to upgrade our bow at Un'Goro Crater.
Kill the "Fire Guard" dogs at Fire Plume Ridge to loot [Searing Heart].
You can find the Spawn Location of Simone here.
You can find the Spawn Location of the Fire Guard dogs here.
Klinfran the Crazed is our next target.
[Might and Magic] > Worldmap > Azeroth > Eastern Kingdoms > Burning Steppes
Kill him and loot his head! Don't forget to pick up every [Heart of the Fiend] you find.
You can find the Spawn Location of Klinfran the Crazed here.
Time to kill Solenor the Slayer.
[Might and Magic] > Worldmap > Azeroth > Kalimdor > Silithus
While killing Solenor we can also start to collect some materials for the upgrade.
Kill the enemies called "Hederine Slayer" to loot [Elemental Heart]. They spawn around Solenor the Slayer.
Don't forget to loot the [Heart of the Fiend] if it drops.
You can find the Spawn Location for Solenor the Slayer here.
You can find the Spawn Location for Hederine Slayer here.
The last target on our list is Artorius the Doombringer.
[Might and Magic] > Worldmap > Azeroth > Kalimdor > Winterspring
Make sure to mount up again because he is the furthest away from the four spawn locations.
You can find the Spawn Location of Artorius the Doombringer here.
Once you've collected all 4 heads it's time to head back to the Stairs of Destiny!
Meet Rhok' delar again and finish the quest to receive the "Ancient Runed Etched Stave".
Talk to him again and accept the quest called "A Proper String".
Time to use our [Might and Magic] again! > Raid Instance > Vanilla Raid > Onyxia's Lair.
Follow the road until you meet Onyxia and slay her, Champion!
Retrieve the Item called "Mature Black Dragon Sinew" and head back to Rhok' delar.
Finishing the quest will reward you with "Enchanted Black Dragon Sinew" to form our Legendary bow.
Right-click the Item "Ancient Rune Etched Stave" and you will form "Rhok' Delar, Longbow of the Ancient Keepers"!