Okay, so for some reason, when I read the phrase "birthday fic" my brain went right past "a fic set during Luthor's birthday" and instead landed on "Happy Birthday, Lexie! Here's a fic just for you!" So I wrote
This and was going to tell you about another fanfic called "Inviolate" (not mine, but very recommended if you have a partly decent familiarity with DC Comics) which always felt to me like the kind of story Lex would write himself (granted, Superman gets off easy, but it does have Lex single handedly taking on multiple Green Lanterns while armed with just a teddybear).