All 3 Uses of

crux
in
Harry Potter (#7) and the Deathly Hallows

Half an hour later, with the tent packed, Harry wearing the Hor-crux, and Hermione clutching the beaded bag, they Disapparated.†
Chpt 19

The idea of the locket being stolen was absurd...As for the school: He alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumed the deepest secrets of that place...And there was still Nagini, who must remain close now, no longer sent to do his bidding, under his protection...But to be sure, to be utterly sure, he must return to each of his hiding places, he must redouble protection around each of his Hor-cruxes...A job, like the quest for the Elder Wand, that he must undertake alone...Which should he visit first, which was in most danger?†
Chpt 27

Definitions:

(1) (crux)
the most important or difficult part of a problem, argument, or situation
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Cruces means crosses in Spanish, so it has worked its way into many English proper nouns, such as Las Cruces, New Mexico.