What happens if I link a Nintendo Account and then decide I want to delete the Nintendo Account?

All Fire Emblem Heroes data (including Orbs you've purchased/obtained) up to the moment you delete your account will be deleted.
If you want to use Fire Emblem Heroes again, you will have to delete the app and reinstall it. You can then start using Fire Emblem Heroes from scratch.

Note: After you delete a Nintendo Account, if you reactivate and relink your Nintendo Account within 30 days, your Fire Emblem Heroes data will be restored.