Lucem is the representative/entity of light. However, Lucem isn't always the good entity. Sometimes the two entities, Darkness and Light get swapped around and whenever that happens, Darkness is the one you want to trust.
Just to clarify this quickly if you're not a Triangulum, Darkness and Lucem hate each other. Darkness a.k.a Tenebrae and Lucem fought each other for millions of years, neither one of them defeating the other with their strong powers. But one mysterious day, Lucem disappeared and was never found again by anyone. Without Lucem, Tenebrae roamed around for many years causing chaos and killing the innocent until one day he too mysteriously vanished.