Manga with supernatural settings and two brothers trying to find their parents' killer
Found it through here... Figures I post something and then find a tagline and remember it was mostly based on Irish mythology.  The manga is called AR∀GO.  The spear was Brionac not the Spear of Longinus. Here is the summary for it:


  The story centers around two twins, Arago and Yuan, whose parents were slaughtered by monsters. Years later, the vengeful Arago crosses paths with Yuan, who has since become a police officer, just as blood begins to flow again in the streets of London.