Under the Shadow is one of the Scourge Invasion quests. This quest must be completed in order to purchase items in exchange for Necrotic Runes from any of the Argent Quartermasters at Light's Hope Chapel or the Argent Dawn camps found in capital cities.
Check your map to find an area under Scourge attack. Go there and damage a necropolis by defeating the Scourge around them. Return 10 of their Necrotic Runes to Commander Thomas Helleran at Light's Hope Chapel in the Eastern Plaguelands.
The floating necropolises of the Scourge plague the lands of Horde and Alliance alike. Tanaris, the Blasted Lands, Winterspring and the Burning Steppes are blanketed in the shadows of the flying fortresses.
Only through our combined efforts will they be turned away.
The necropolises are supported by magical circles at various points in the invaded areas. We have determined that destroying the mindless hordes that surround these circles will destroy their defenses.