The Tiefling's Family Secret

From the introduction:

A tiefling sage, Devlin Dross, has discovered a disturbing intimation in the annals of his family. The chronicles speak of gaining power and learning secrets in sideways, tangential terms which lead Devlin to fear some dark secret kept hidden even within the family itself. He has uncovered references to a private retreat where family members would go for these purposes and thinks it may be a library of forbidden lore which needs to be sought out and destroyed. He believes he will need an escort of adventurers, as the histories also obliquely mention guardians and wardens he does not think he will be able to handle on his own, as he has no real skill in combat.