Seer

"They're supposed to be able to see the future, like prophets?"

- Merlin to Gaius about Seers

Seers are individuals with the ability to see the future. Some Seers are able to foresee the future in their dreams, while others can see visions through other means, such as Scrying.

Gift of Prophecy
"Some aren't even aware that what they see is the future. It comes to them in their dreams."

- Gaius to Merlin

Seers with the gift of prophecy are able to see visions of the future in their dreams. According to Gaius, it is an innate ability which cannot be learned; those that have it are born that way. It also appears to be involuntary (The Gates of Avalon).

Morgana Pendragon possessed this ability. She believed her dreams to be little more than nightmares at first, an idea that Gaius encouraged because the gift of prophecy was too close to the work of magic in Uther's eyes (The Gates of Avalon).

Some of Morgana's more noteworthy visions include:
 * Sophia attempting to kill Arthur (The Gates of Avalon).
 * Arthur being attacked by the Questing Beast (Le Morte d'Arthur).
 * Guinevere being crowned Queen of Camelot (Queen of Hearts).
 * The Battle of Camlann (The Darkest Hour).

Morgause may also have been able to see the future in her dreams, as she once told Morgana that she, too, suffered from nightmares. However, she may have been referring to normal dreams, without the prophetic element (The Sins of the Father).

Nimueh also claimed to have powers of prediction, though it is unknown whether she was referring to the gift of prophecy (Le Morte d'Arthur).

Scrying
"Look into them, Emrys. Really look. Much will be revealed."

- Taliesin to Merlin about the crystals

Scrying is a magical practice that allows a Seer to see visions in reflective surfaces such as water or crystal. It can be used to view events in the past, present, or future.

Nimueh often used her scrying fount to spy on her enemies (The Mark of Nimueh, The Poisoned Chalice).

Morgause used a crystal to observe both Camelot and Cenred's army (The Sins of the Father, The Tears of Uther Pendragon).

Merlin was able to see visions in both the Crystal of Neahtid and the Crystal Cave (The Witch's Quickening, The Crystal Cave, The Diamond of the Day). He also saw a vision of Arthur's death in a pool of water (Arthur's Bane).

The Seer Taliesin was also said to have divined prophecies in the crystals of Crystal Cave (The Crystal Cave).

The Disir were said to use an ancient pool to divine the will of the Triple Goddess. The pool was fed by a sacred stream at Caerlanrigh, the source of which was the Grove of Brineved (The Disir).

Unspecified
The dragon Kilgharrah often advised Merlin about his destiny. He knew many ancient prophecies and was aware of certain future events (The Dragon's Call).

The Druids also had knowledge of prophecies and appeared to be aware of Merlin's destiny (The Beginning of the End).

The Fisher King knew about many future events. He was aware that Merlin would come to the Perilous Lands and awaited his arrival for hundreds of years (The Eye of the Phoenix).

The Cailleach warned Morgana that Emrys would be her doom (The Darkest Hour).

The Dochraid was aware that it was Morgana's destiny to bring back the Old Ways (Lancelot du Lac).

With his dying breaths, the Seer Lochru warned Merlin to beware of Arthur's bane. Kilgharrah later identified him as a Vates, a Druid Seer whose power of prophecy was unrivalled, even by a High Priestess (Arthur's Bane).

The Diamair was known as the key to all knowledge, including knowledge of the future (Arthur's Bane).

When Arthur, Merlin, and the Knights of the Round Table set out to rescue Gwen from the Dark Tower, Queen Mab warned Merlin that one of their number would not return. Her prediction later came true when Elyan was killed by Morgana's enchanted sword (The Dark Tower).

For hundreds of years, the Catha guarded an ancient prophecy foretelling the Battle of Camlann. When the great battle drew near, Alator ordered Finna to deliver the prophecy to Merlin. He also entrusted her with a personal message for the warlock advising him not to trust Mordred (The Kindness of Strangers).

Potential Consequences
"To change the future is no simple matter, Merlin. To do so is fraught with danger."

- Kilgharrah to Merlin

Though useful, the power of foresight is not always helpful or harmless. It has been shown that more often than not, the very people who have visions are the ones who cause them to come true. For example, Merlin made his own vision come to pass when he set off a chain of events by accidentally injuring Morgana (The Crystal Cave).

According to Gaius, the future is yet unshaped and there is nothing on earth that can know every possible future (The Witch's Quickening). Similarly, Morgause once told Morgana that there are many futures, some shrouded in mist and others clear as cut glass (Queen of Hearts).

However, some viable futures were stronger than others. When Morgana's healing bracelet failed to stop her vision about Gwen becoming Queen, Morgause warned her that it was a prophecy too powerful to ignore and told her to do whatever she could to prevent it (Queen of Hearts). In the end, however, Morgana was the one who indirectly made her vision come to pass by setting off a chain of events that began when she enchanted Gwen to betray Arthur with Lancelot (Lancelot du Lac, The Sword in the Stone).

Trivia

 * It is unknown whether scrying is an ability possessed only by Seers or a skill that any sorcerer can learn. However, Taliesin once stated that the future is hidden to all but a very few, which might imply that only certain people are capable of seeing the future under any circumstances (The Crystal Cave).
 * There are enchantments that can prevent a Seer from dreaming, such as the one Morgause placed on Morgana's healing bracelet. However, they cannot stop prophecies (The Sins of the Father, Queen of Hearts).
 * Under certain circumstances, it is possible for non-magical people to have visions. Arthur, for example, was able to transcend his body so that a quest would be revealed to him (The Eye of the Phoenix).

In the Legend
The gift of prophecy was one of Merlin's chief abilities in Arthurian Legend. In fact, in many early versions Merlin was depicted not as a magician, but as a prophet and a wild man of the woods. Morgan le Fay, meanwhile, was often portrayed as a highly skilled healer. In many versions of the legend she took Arthur into her care after he was wounded in the Battle of Camlann.

Interestingly, the series appears to have reversed this dynamic. Morgana possesses the gift of prophecy and can see the future in her dreams, while Merlin is the ward and apprentice of Gaius, Camelot's court physician. However, he later learns to see the future with scrying.