Sure, there is an Overlord manga. It expands on the story and characters from the original light novel series, giving fans another way to enjoy the world of Overlord.
Overlord is actually both. The light novel provided the foundation and the manga was created based on it. While they share the core concepts and characters, there can be differences in the presentation and emphasis in each medium.
It depends on your preferences. The novel offers more detailed world-building and inner thoughts of characters. The manga has vivid illustrations that bring the story to life visually.