Telepathy

Telepathy is a form of magic that involves transmitting speech into a person's mind. This power allows magic users to communicate with each other without being overheard by other people. Skilled practitioners of this power rarely spoke with their mouths. Telepathy is extremely common among the druids. Others like Merlin, Mordred, Morgana, Morgause, Kilgharrah, and Balinor were also capable of using this power. Since Merlin wasn't able to warn Daegal with Telepathy, it can be assumed that people without magic can't be contacted with this power at all.

Telepathy has been used the following times:
 * Kilgharrah used telepathy to contact Merlin after his arrival in Camelot to tell him of his destiny (The Dragon's Call).
 * Mordred used this power to call Merlin for help after his father was captured and later both he and Merlin telepathically talked to each other (The Beginning of the End).
 * Mordred used telepathy to talk to Morgana during her stay with the Druids (The Nightmare Begins).
 * Mordred used telepathy to direct Alvarr to Morgana's chambers (The Witch's Quickening).
 * Morgana presumably used telepathy when she was dying, as Morgause heard her cries (The Fires of Idirsholas).
 * Mordred again used this power when telling Merlin that he would never forgive or forget his betrayal (The Witch's Quickening).
 * Kilgharrah telepathically shouted at Merlin when demanding to be released (The Witch's Quickening).
 * Despite dying, Balinor used telepathy to tell Merlin how to use the power of a Dragonlord (The Last Dragonlord).
 * Morgause used telepathy to contact Morgana when she had a vision of Gwen becoming Queen of Camelot (Queen of Hearts).
 * Iseldir used telepathy to tell Merlin that he was now the Cup's caretaker (The Coming of Arthur).
 * The Druids used telepathy to contact Merlin when they assisted him in finding the tomb of Ashkanar (Aithusa).
 * Merlin tried to warn Daegal with his mind to hide when he spotted a group of bandits in the Valley of the Fallen Kings. However, since the boy was not a druid, he couldn't use telepathy (The Hollow Queen).
 * Mordred and Kara used telepathy to communicate with one another. When his love interest was hanged by Arthur, Mordred emitted a scream that pierced Merlin's mind (The Drawing of the Dark).
 * Morgana, while taunting a powerless Emrys in the Crystal Cave, used a form of telepathy which made her voice sound as if it was coming from many different directions (The Diamond of the Day: Part One).