Cerdan was a druid master and a guardian of Mordred. It is possible he was related to him, and was most likely his father. He and his apprentice Mordred went to the market and tried to buy something from a stall man but the man betrayed Cerdan, apologising as the guards pursued him. Cerdan told his apprentice to run after he used magic to close the gate. He was shortly after captured and executed by Uther. Later, while Morgana and Merlin took care of Mordred, they saw Cerdan get decapitated. Mordred reacted to this with a sonic scream that shattered the glass of Morgana's mirror, suggesting his strong bond with Cerdan that could be, but is unconfirmed to be by blood. Later Gaius indicates that Druids take children showing magical abilities as apprentices, indicating Cerdan is Mordred's master (The Beginning of the End).
While Cerdan is an original character and possibly the father of Mordred, in the legend Mordred's father in most sources is King Arthur or in some other tales of the legend King Lot while his mother was either Morgan le Fay (via Arthur) or her sister, Anna Morgause which isn't the case in the show.