The Tomb of Sardar is found in Act II of Diablo III, at the northern end of the Dahlgur Oasis.
A lorebook titled The Musings of Sardar is present, which is required for the Judge of Character achievement.
Inside, the Sardar's Treasure event starts, and Sardar himself can be found in the end of the dungeon (in his undead form).
To open the doors (both entrance and the vault), player(s) must find and turn the two levers: one outside the cave and one in its farthest end.