Warning: The following text contains spoilers relating to the plot of Prophecies.
The Temple of Ascension is a temple built in the southwest of the Crystal Desert by the Elonians. In the Dunes of Despair mission, the party is required to reach it, and take over the Throne of Pellentia inside the temple. Then they need to defend the Ghostly Hero upon the throne from the waves of Forgotten and Enchanted attackers.
The Temple was built by Elonians in their quest to achieve Ascension.