Nymph's "Unearthly Grace" feature
 

I'm looking to make a creature that gets its charisma bonus to saves and armor class like a Nymph does, where would I find that monster ability, or how would I go about creating it?

thank you.

Might be worth taking a look at the Paladin's Divine Grace and Smite Evil class features.

You can launch the editor from the Tools menu.

Then, create a new file in the editor, and switch to the "Race" grouping. On the "Racial Special" tab, press the "New (Copy)" button, and choose "Unearthly Grace" from the list of abilities there. If it gives you a message about not being able to save the copy, then you'll also need to create a "New (Blank)" racial special, and copy the Eval Script and everything else from the copy to the new item - then you can delete the copied item and save the new one.

Oh, well, y'know, if you want to make it easy... :p

How would I add the Nymph feature of getting druid spells at X-1 where X = racial hit dice?

Set the SplLookup.cHelpDrd onto the race, and then set the cMagicLev field of the race to racial HD - 1.