Seeking Disclosure

Find The Elder Scroll (Dragon)
The Dragonborn may well already have the Elder Scroll (Dragon) by finishing the main dragon-related quest line. Otherwise Discerning the Transmundane will make this Elder Scroll available.

Find The Elder Scroll (Blood)
Follow Serana's hunch that her mother probably is in possession of the Elder Scroll (Blood), in Chasing Echoes and the follow-up quests.

Bring both Elder Scrolls to Dexion
Once both scrolls have been obtained, return to Dexion Evicus and speak to him. Be he at Castle Volkihar (siding with vampires) or at Fort Dawnguard (siding with the vampire hunters). Alas the old man has gone blind after having read the Elder Scroll (Sun), so can no longer directly help, but he suggests the means by which the Dragonborn can read them. He also returns the Elder Scroll (Sun) to the Dragonborn Offenlegung erwünscht