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There are a few endings for La Pucelle, but the canon one seems to be the hidden ending, in which Prier becomes so powerful she "falls". She is declared the Overlord of her worlds hell because she is so strong.
It was supposed to be a joke because if you grinded enough to get that ending, you are that strong. But it seems the joke was so funny it became truth.
Prier's miracle attack in Disgaea is actually the various miracle attacks you could make in La Pucelle by trapping enemies in the geo-grid system (hence the square) and setting off the reaction.