The Ritual

Ritual needed to replenish stores of Magic in Fairies so that they're "running hot" and can perform magical tasks.

According to the The Book, fairies must perform a specific task to refill their magic. A seed (such as an acorn) must be buried under the shade of an ancient oak tree on the curve of a river under the light of a full moon. Places such as these are rare now due to logging efforts and other human interference, not to mention the weather. Therefore, performers of the Ritual can be pinpointed, as in the case of Artemis Fowl.