Ascension Quest: To become a Crusader, you must first become a Paladin. Once you have ascended to your new role, the Celestial Order will ask you to place a Crystal of Clarity within each of the three Shrines of Celestial Power. The first shrine can be found on the island in Lost Lake, the second among the Harrow Ruins west of Lake Dread, and the third within the Forbidden Lands south of Skuldoon (in the eastern deadend along the Southern Boulders border).
Once all three crystals have been placed, return to the Celestial Order to receive yet another quest. This time, you must travel to the Vale of Rune to defeat the Ancient Evil summoned by the wizard Volgar. Once you have confronted the foe, return to the Celestial Order to ascend into the Crusader class.