What do you do after you gather all light for Nexus?

[AngelMoon]

Figured out my problem

[Lemuria]

That depends. If you're a Paladin you need to not only have the Curtana fully soul-charged but also the Holy Shield. If you've only charged one of them, you cannot continue. Assuming you've done that, or you've charged another weapon, it's a simple case of talking to the NPC and choosing relic enhancement, selecting your weapon and getting it enhanced. You CANNOT be equipping it when you do this.

Note that this isn't done as some quest option. It's done under the other dialogues. The next quest will become available once you have your Nexus in hand.