A Malum Librum is a portal to an alien world disguised as a book. These worlds are commonly full of evil monsters, but also some good ones. Many great adventures have begun with an unwitting child stumbling upon one.
Poor Mikey found a Malum Librum last month and it took him two weeks to get home! He missed our date, too. But he brought back a souvenir for me: a lich's soul jar. So yeah, it's all good.
is a Latin phrase meaning wrong or evil in itself. The phrase is used to refer to conduct assessed as sinful or inherently wrong by nature, independent of regulations governing the conduct. It is distinguished from malum prohibitum, which is wrong only because it is prohibited.