Killed it again in DSP's cave and it dropped the following 8 items:
Stick of Mastery
Loop of Energy
Grub Worm Guts
Book of Sorcery
Shadowed Crystal
Mark of Charm
Page of Prose
Ocher Gem
Seems like out of ordinary mobs in DSP that have "frenzied", "Darkened", "sturdy", "dire" etc. all have a chance to drop these quest items.