With that out of the way, the Obsidian Sanctum is home to Sartharion the Onyx Guardian and a trio of black drakes. They look more purple, perhaps lavendar, but we'll stick with black. The Drakes are mini-bosses, only with a few minor mechanics to learn and understand. Killing each one of them removes an ability from Sartharion himself, who you attack last. There is some trash between mini-bosses, but it's minimal.
The bosses are currently somewhat undertuned, but they're not going to be particularly difficult for a reason. Downing them one by one is not the challenge of this raid zone. Like Zul'Aman, it has an additional challenge for you to tackle after you've killed everything on "easy mode." You can pull Sartharion before all of the Drakes are dead for bonus loot. That bonus loot includes a Black Drake Mount