How to unlock: Talk with Klin'Kar in the Valley of Dead Heroes (M12.1) or reach the whisperlock found in the final part of Necropolis (M13.3).
This quest involves getting through a whisperlock found in the Necropolis (M13.3). The whole procedure is mandatory, as this unique safeguard blocks your access to The First Castoff's tomb, which is one of the locations visited during the main quest.
You can learn more about the whisperlock by talking to Klin'Kar (M12.1), but only if you've agreed to help The Children of the Endless Gate in finding Memoralists. Additionally, you can speak to Motley once you've reached the Necropolis (M13.1). If you've decided to fight against the cultists, you'll learn about the lock once you stand before the terminal itself (M13.3).
The way to unlock the whisperlock will differ depending on which faction you've chosen. If you're helping The Children of the Endless Gate, visit the Memoralists camp found in the western part of Valley of Dead Heroes (M12.6) (the upper wall can be accessed by using the dark minnim (M12.2)). Talk to Ronos Cai'Sul and convince him to share the password (Guni) with you. Once you've acquired the password, either give it to Motley in Necropolis (M13.1) or ignore him and reach the lock on your own.
If you've decided to help the persecuted Memoralists, your job will be to eliminate Motley and other cultists found in the area. Defeat them all (they shouldn't pose a serious threat) and speak to Talira Kyo. She will help you unlock the terminal used to choose vaults (the list of all accessible tombs can be found in the key for the Necropolis map (M13)