Invisible Stalker
This NPC can be found in Borean Tundra (70), Dragonblight (32), Stormwind City (16), Crystalsong Forest (14), Zul'Drak (10), Sholazar Basin (6), The Storm Peaks (6), Grizzly Hills (5), Icecrown (5), Howling Fjord (4), Wintergrasp (4), Strand of the Ancients (3), Halls of Lightning (1), Durotar (1) and Magisters' Terrace (1).