You go in the palace in Lut Gohlein, go all the way down until you reach a portal. Enter it, and find The Summoner. Kill him (though when I did this I died without hitting him once, and when I went back after "finishing" The Summoner quest, he [and the monsters] were gone!) and then read the journal. Then go in the red portal to the Canyon of the Magi. Activate the waypoint and then scout along the perimetre, until you find the tomb (it has a triangle on it). That's the real tomb (though it's advised that you are lvl 21+ before entering so that you can kill the boss [Duriel], just train in the fake tombs).