A Dragonlord is a warlock who possessed the rare ability to tame and converse with Dragons. According to the Old Religion, Dragonlords were brothers to dragons through a connection of the soul, with the abilities being hereditary. This talent was passed down from father to son.
Following the Great Purge, when Uther Pendragon destroyed all creatures of magic, the Dragonlords were believed to have been wiped out. But one, Balinor, avoided persecution by escaping from Camelot with the help of Gaius. He went to stay in the village of Ealdor. Whilst there, Balinor met and fell in love with Hunith, but was forced to leave her when Uther was informed of his whereabouts. Several years later, Balinor discovered that whilst with Hunith he had conceived a son, Merlin, who inherited his father's Dragonlord abilities when Balinor died. Merlin then used this ability to defend Camelot from Kilgharrah (The Last Dragonlord).
Once they inherited their gift, all Dragonlords were able to speak to any Dragon in the language shared by their souls. The Dragonlords could use their abilities to force tasks upon Dragons, as they had no say in the commands they were given. This communication appeared to be, at least in some part, telepathic as Merlin and Kilgharrah were able to hear one another over long distances (The Last Dragonlord, The Crystal Cave). These abilities also worked on Wyverns, distant relatives of the Dragon (The Eye of the Phoenix).
They even have their own subvarity of magic, Dragonlords magic.
By forcing a Dragon to submit to them, Dragonlords can then ride the creature, giving them control over its natural might (The Tears of Uther Pendragon).
Only a Dragonlord can cause a Dragon egg to hatch, calling the mature embryo in the egg from its dormant state and out of the shell with its name (Aithusa).