Trinity is one of the best items in the game. It's not because of it being good at any one thing, but because it's good at so many things with only one slot taken up is what makes it god-tier. It has one of the highest gold utility in the game, and it can provide a huge power spike on the right skill set. Lucien is one of those champions which benefits from it. After all, if you're going to already weave attacks in between spell casts, why not get a 150% bonus to damage in addition to the passive? That's a damage spike you can't get with any other item.