Having Corthana's spellbook in hand has me convinced that our next step might be to seek out relics from all of the individuals who crusaded against Regulos. If we had them in our hands, perhaps we could learn what exactly brought about his undoing.
Ultimately, we'll need to know how to defeat his spirit and not just his body, but knowing how they killed the black dragon will help. Speak with Rahn about Won Odego, Dacia about Anan Mkhai, Brother Damon about Nyol, and Uriel about Neddra.
Good. Our scholars will get to work putting these together. I know Sylver is particularly excited about what you have brought him.