Guide: The Black Drake will drop from Sartharion on Legacy Raid Size 10man within the raid, Obsidian Sanctum. It drops on a 100% drop chance but only if you defeat Sartharion with the three Twilight Drakes, Tenebron, Shadron and Vesperon still alive when you engage Sartharion. If any of these drakes are killed before you engage Sartharion then the drop chance will become 0%.
The Twilight Drake can also be obtained from Obsidian Sanctum in the same way but on Legadcy Raid Size 25man.
The entrance to Obsidian Sanctum can be found within the lower section of Wyrmrest Temple, you can reach the lower section by entering the large crack in the ground north of Wyremrest Temple. Once inside the lower section of Wyrmrest Temple, walk straight and you will see the entrance to Obsidian Sanctum.
To get the Black Drake, you must complete Obsidian Sanctum on 10 man difficulty. To set the difficulty, right click on your character portrait, go down to Raid Difficulty, set it to either Normal or Heroic, this part doesn’t matter as long as it isn’t on Mythic. Then under Legacy Raid Size, set it to 10 player.
Once inside you will see Sartharion immediately in the center of the room and around the room, the three Twilight Drakes. You are free to kill the trash in this raid but you MUST NOT kill the Twilight Drakes before engaging Sartharion. You can kill them during the fight but just not before the fight (although chances are you will defeat Sartharion before the Twilight Drakes engage in the fight).
The fight is very trivial for a character above level 100, just engage Sartharion and damage him until he dies. All of the trash that is still alive will engage but they do irrelevant damage for a character of level 100 and above.
Once the fight is over, a chest will spawn in the center of the room, chances are it is burred below Sartharion’s corpse and all the trash, so you may need to loot them first before you can access the chest.