What exactly is a complex dermoid ovarian cyst with an enhancing mural nodule? And how serious is it?

Not cancer. A dermoid cyst is a benign type of turmor containing multiple types of tissue and has a characteristic appearance under sonography. It will always have a complex appearance and the mural nodule is simply part of that. Most dermoids gradually get bigger and have to be removed at some point, but if it remains stable it could be watched. Surgery can be done day surgery with small incisions.