Seer

A Seer is an individual with the ability to see future events in dreams or visions, known as the Sight. Morgana of Camelot possesses such an ability, and foresaw the treachery of Sophia with it (The Gates of Avalon).

The sorcereress Nimueh also claimed to have the power of prediction (Le Morte d'Arthur).

Another known Seer is Taliesin, who was the keeper of the Crystal Cave.

Morgause might have been a Seer as well, as she told Morgana that she suffered from nightmares too. It is, however, possible that she just had normal dreams, without prophetic element. (The Sins of the Father).

According to Gaius, the gift of prophecy is considered a form of witchcraft in Camelot. As a result, he has kept Morgana's ability a secret even from her, and instructed Merlin to do the same.

The ability to See appears entirely involuntary. Morgana says as much to Merlin when she indicates that she belives that magic may choose the person and not the other way around (The Beginning of the End).

The dragon, Kilgharrah, was a seer and often spoke to Merlin of his destiny to stand by Arthur when he'd eventually become king.

The Druids had a similar gift, being aware of Merlin's destiny in Albion.

The Fisher King also was aware of Merlin's destiny, as well as Morgana's future crowning as queen, thus he gave Merlin the Avalon Vial to use it then.

Besides foretelling the future, Seers appear to have similar abilities to those with other magic skills. For example, like Merlin, Morgana can communicate telepathically with others, such as Mordred, and can cast spells.

The power of foresight is however not always helpful or harmless. It has been shown on more then one occasion that the very people who have a vision are the ones who are making them come to pass. Merlin made his own vision come true in The Crystal Cave. Gaius claimed that events seen in visions don't always come to pass, as a seer is just able to see one of the many possible futures. Morgause told Morgana a similar thing when she said that there are many futures in this world, some shrouded in mist and other as clear as cut glass. She warned her that the dreams she kept having about Gwen becoming queen where prophecies and she had to do whatever she could to prevent them from happening (Queen of Hearts). However, in the end Morgana was the one who indirectly made this future come to pass, by a consecutive chain of events that began by her enchanting Gwen to an adultery with Lancelot.

A seer's dreams can be stopped from not making him/her sleep at night if an enchanted bracelet is worn, but this only stops simple visions and not prophecies.