The mysterious letter is found during Missing, Presumed Death by searching the bench next to the Temple of Saradomin in Paterdomus. It is used to help Brother Samwell in finding out who killed the monks.
It is presumably directions from Sliske to the wights, who are trying to lure Icthlarin to the area to deliver an invitation to Sliske's ascension to godhood.
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Your orders:
S
