The Ijiraq were once used as spies and assassins by those who controlled them. They appear as half-man, half-mist but can take almost any form, making them particularly deceptive. When encountering one, you will see it out of the corner of your eye, then if you try to observe it directly they are completely elusive. If and when an Ijiraq allows you to see him, it is only in the moment before he intends to slaughter you. Luckily, no man alive knows the means to command an Ijiraq.
Examples of Ijiraq may be read in Seeking Solace, where Aria and Dane encounter one in Novodevichy Cemetary or Hunt the Hunter in which Michael and Dr. Weston go on the search for the Ijiraq loitering around Mecca.
Additional information may be found on the forums' worldbuilding thread.