*Research on Undead slaying*
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Aeric enters the library and spends time specifically searching for various types of more powerful undead (vampires and Liches, and other forms of powerful undead) and wishes to find out about them and what it takes to kill them.
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Then he should search through the posted books, many of which already discuss this very question. IE: Before you post, its easier to do a forum search so DMs don't spend time repeating things others already asked about.