I think when someone is trying to summon HWWB, they call him something like "darkest shadow."
Also, in Small Favor, when Eldest Gruff is chatting with Harry after killing Magog, he refers to the Denarians as "Servants of the darkest shadow."
I just checked. Gruff actually does say it in SmF. My text search is failing to find that string. “Hast thou asked why the wicked Queen of Winter would wish thee to prevent Marcone from being taken by those servants of the darkest shadow? Why she who embodies destruction and death would wish to protect and preserve the Archive?” - Butcher, Jim (2009-03-03). Small Favor: A Novel of the Dresden Files (p. 374). Penguin Group. Kindle Edition.
Now we just have to question whether he's referring to Lucifer, or referring to Nicodemus controlling the other Denarians and having them do his dirty work. One's the master of shadows, and the other is the Prince of Darkness.
As for the summoning of Behind, here's what Marge says: “While here we wait, O hunter of the shadows! We who yearn for your shadow to fall upon our enemy! We who cry out in need for thy strength, O Lord of Slowest Terror! May your right arm come to us! Send unto us your captain of destruction! Mastercraftsman of death! Let now our need become the traveler’s road, the vessel for He Who Walks Behind!” - Butcher, Jim (2010-03-03). Blood Rites: Book six of The Dresden Files (p. 346). Roc. Kindle Edition.
...
But this text search thing is driving me nuts. I can even search "servants of the darkest", and it'll find it, and "shadow?". But if I search "darkest shadow", "darkest s", or even "t shadow", it comes back with 0 results.
Now I'm questioning all of the searches I've done.