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The Mark of Vashj is a spell cast on the player with the completion of  and returning to Skar'this the Heretic. With this spell cast on the player, they are allowed to enter Serpentshrine Cavern, the lair of Lady Vashj. As of Patch 2.1.2 this is no longer needed to enter SSC.
With Patch 2.3, players who complete or have completed the attunement process for SSC/TK will receive the title Champion of the Naaru.