'Pokemon Brilliant Diamond and Shining Pearl' Cresselia Guide: Where, How to Catch this Generation IV Legendary Pokemon
In Pokemon Brilliant Diamond and Shining Pearl, players are given the chance to catch Pokemon in the Sinnoh region, even if it is a Legendary one like Cresselia. But in order to catch it, they must make it appear in the region first by accomplishing a task.
Source: gamenguide.com