"Morgana finds a powerful ally in the Southron warlord Helios , as the drama continues. Together they hatch the perfect plan to force Camelot to its knees... With the Kingdom - and Arthur - preoccupied by the arrival of the beautiful Princess Mithian , it falls to an absent friend to raise the alarm. But with past wounds still raw, can love really conquer all when Guinevere returns? Or is a deadly arrow destined to fly straight into the heart of Camelot? "

http://merlin.wikia.com/wiki/Episode_4.11
http://cultfix.co.uk/merlin-series-4-episode-11-details-15019.htm

no to już wiemy skąd dama w bieli :D