Conjunction

A conjunction is a place in space or in time when the laws of magic are bent enough to allow actions that are normally impossible. It is first mentioned in Cold Days.

Description
Though as yet unexplained, conjuntions are implicated in a number of magical phenomena. Once a year, there is a conjuntion when immortals are vulnerable, Halloween, which is the only time when a Darkhallow can be attempted. According to Bob, Cowl knew it, explaining why the Kemmlerites attempted a Darkhallow in Dead Beat.

Darkhallow
In Dead Beat, necromancers, the Heirs of Kemmler—Cowl, Corpsetaker and Grevane—raced each other to be the first to attempt a dark spell called the Darkhallow on Halloween night to make themselves into a godlike being.

Dead Beat
In Dead Beat,

Cold Days
In Cold Days, Harry Dresden had to swear on his magic to build Bob a new house if he told him how to kill an immortal. He did. And Bob told Harry that there is a conjunction in which immortals are vulnerable, once a year—Halloween.