you have to talk with jibril about a book on this "beast" she is the demon that is in the library. you can find the library by going through the quest line of sending adventurers out there to stop the demon through the work option. after a while you will have to go do it yourself since no adventurers are capable of taking jibil down. also, DO NOT CHOOSE TO FIGHT HER WHEN THE UNDERTALE PART HAPPENS. no matter what you do, you WILL lose. it will send you to fight her with your group in physical combat and you might as well shove a tube of lube up your a** to make it less painful when she inevitably shoves her boot up it afterward. I dont think she was ever intended to be fought in physical combat. like, a trillion health or something with a trillion defence. Only click ACT. and select the move option. it will have you dodge attacks, but its the only way to beat her, and thats at her own game. after that, and after allowing her to stay there as the one who runs the library, you will have to go back and you will ask if theres a book on this "beast"