As John said, we’re trying to research this with more authoritative sources than are usually found free on the web. Some brilliant research and theorizing is being done, but here at the Lexicon we want to be cautious and make sure we’re giving you the best tools we can.
One area of confusion for a lot of people is “is ‘Hallows’ a noun or verb?” Many fans are going to one of the free dictionaries on the web and they are inadequate for such old-fashioned words. I consulted the mother of all English dictionaries, the Oxford English Dictionary and found that hallows can be either a noun or a verb, but that only one form of usage makes sense in the context of the stories.
Deathly as an adverb: Resembling death (“deathly cold”).
Hallows as a verb: To make holy or set aside for God (“Bring their candles to be blessed and hallowed”).
My interpretation? A blessing ceremony that turns deadly or looks like it does — which seems too overtly religious to me.
Deathly as an adjective: Causing death, resembling death or pertaining to death (“deathly mists,” “deathly stillness” and “His wounds, many and deathly”).
Hallows as a noun: A saint (All Hallows=All Saints), a saint’s shrine or collection of relics (can also be pagan relics). Note: The noun is usually plural.
My interpretation? A powerful place or collection of powerful objects that appears death-like, causes death or has power over death.
Another curious meaning for ‘Hallow’ and ‘Hollow’ is that it is an old-fashioned way to shout hello, especially while hunting. I am certain that this is not what Jo has in mind.