Task Finder
...
The Assassin's Prayerbook is one of the many Key Items found throughout Elden Ring, located behind the second Imp-sealed door on the lower level of Roundtable Hold.
The Assassin's Prayerbook is a Key Item that will unlock the ability to learn the following incantations.
This Prayerbook can be found on the lower level of Roundtable Hold, beyond the second Imp-sealed door you'll come across after unlocking the first.