I'm thinking more and more that the right approach in monster-ology lies between Flame Princess' "do it yourself, do it all yourself" and conventional D&D's "here, all the monsters are in this book." In fact, my one-armed man chart from earlier in the week comes within an ace of accounting for the hit dice of most existing classic monsters, and is a good template for any others you may care to design.
But what about the stranger stuff, like undead? Well, what makes undead scary and unpredictable is their special abilities, both attack and defense. This suggests a little d6 table with possibilities for abilities that will make each encounter uncertain and memorable. After all, folklore doesn't treat ghosts as a race, but as a class of cursed beings, each with its own story.
A final issue: undead-ism is most often seen as a curse transmitted unwillingly, but the more frightful possibility is that some persons desire the various states of undeath, as a means of achieving desires unavailable to living mortals. It's this potential that I think affords a new vantage point on these venerable villains.
A Quick Glance at a Mythoard Exclusive
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