In Abyss Foscari, at the location of Seal of the Banished, there is a boss-ish creature (Je-Ne-Viv). You need to beat that.
I fought it right after breaking the seal, not sure how that works but all my weapons from the dlc and my evolutions were gone. At some point, Prismatic Missile appeared next to me and kept appearing till i reached max level. (I took Prismatic Missile before I started the fight so not sure if you will receive unless you already found and took it.)