In Conrad's Fate, the Walker is an agent of the Lords of Karma. Although it is the shoddy magic of Uncle Alfred's Magicians' Circle that sets up Conrad's meeting with the Walker, the Walker himself (itself? herself? who can say) is an instantly awesome being. Before Conrad is sent up to Stallery to carry out the circle's assassination plan, each of the members holds the summoning object -- in his case, a wine cork -- and adds some kind of magical oomph to it. Once at Stallery, Conrad is to hold it up and say "I hereby summon a Walker. Come to me and bring me what I need."
The precursor to Conrad's use of this spell was supposed to be a feeling of certainly about who he was supposed to kill. Unfortunately for Conrad, whatever magical devices that were supposed to create this feeling were probably stripped away when Christopher first met him and without mentioning it at the time, removed the tangle of spells that surrounded him.
As a result, Conrad keeps looking in vain for signs of when or if he should use the cork to summon something, and when he meets his sister Anthea in the manor's library, he explains what he's doing there and shows her the wine cork. Impatient with their uncle's tricky little machinations, she agrees with Christopher's suggestion that Conrad just try summoning it at once. So he does so.
The result creates shock and awe in all three of them. The room appeared to be simultaneously just a library, full of library smells and silence, as well as an arctic expanse, full of the smells of intense cold and a wind that you could sense but not feel. They could see a curved horizon in the distance and a figure hurrying towards them. It had dark hair that seemed to be blowing in the insensible wind, and dark, intent eyes, and as it came up to them, it appeared taller than the ceiling, taller than everything. It held out a hand, into which Conrad put the wine cork, and then handed him another object it had been carrying. Once he had it, the Walker, the horizon, and the arctic distances were suddenly not there at all, and the three of them were shaken. It was a very unsettling experience.