The Whispering Blade Chapterhouse is restricted to those who have formally joined the Whispering Blade faction through completing a series of quests. Rossu Morta is an adversary faction. For quest details see Whispering Blade Chapterhouse Access. To use the entrance portal you must be wearing the Ring of the Whispering Blade, however once you have completed the Tracking Grael you can recall directly inside with help of the Whispering Blade Chapterhouse Recall Orb.