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