Warning: The following text contains spoilers relating to the plot of Prophecies.
The Mesa is a giant sacred rock directly east of Augury Rock with an oval shape and a flat top. Hidden inside this rock is a cave where players get to fight their Doppelganger. This is the final test for Ascension.
To gain access to the Mesa, players have to complete all 3 missions in the Crystal Desert, and then talk to Great Ritual Priest Zahmut in Augury Rock. You will be teleported to the entrance of the Mesa where you have to step on 3 runes on the floor to open the gate to the Mesa cave.
Note: If you didn't talk to the priest before doing the missions, you may have to redo them. Talk to the ghosts at the mesa. If one of them says that you need to do a mission, you need to redo it. If they all say you are ready, walk through the rune circle and stand on top of each rune, 1 at a time. The third rune will start a cutscene.
For further details see: Prophecies Ascension guide