A Hallow is a saint. Halloween stems from the word All-Hallows-Eve.
So it seems to refer to some sort of anti-holy group
The word deathly is what bothered me about the title. Its too blunt. I guess Deathly was used instead of Deadly because it implies that death encircles the group or whatever the hallows are. Deadly would just mean they kill people...
But thruthly I really dont care about the title, but for the content of the book.