Obsidian Eradicators are mobs inside the Temple of Ahn'Qiraj. They pull singly. They can drain mana from all mana users in range. It seems that they choose 16 candidates to drain mana from as soon as aggro is acquired. They will cast an area effect Shock Blast on everyone within 30 yards (6.5K nature damage) if their mana is allowed to reach 100%, so they must be drained of their mana with Mana Burn, and Drain Mana. Viper Sting is not recommended as a mana drain, because it will take up valuable Debuff real estate, even if you only have 3 warlocks and 3 priests draining his mana. Viper Sting is simply not needed.