Sidhe Magic

Sidhe magic is a form of magic practiced by the Sidhe and in part by their loyal servant, Pixies. Sidhe magic has spells of its own, different from those used by the other magic users (in the show, the Sidhe spells are in Old Irish, while all the others are in Old English). The Sidhe are masters of enchantments and their magic is very powerful. At the very heart of Sidhe magic lies the power over life and death, as it's written on their staffs.

However, other warlocks or sorceresses can learn to control and use Sidhe magic. For example, Merlin was able to harness a Sidhe staff, stolen from Sophia, to his own magic. The Witches of Meredor, considered by Gaius the worst recipes writers, created a potion that could force the faerie out of a changeling, and thus control one of the most important characteristics of Sidhe magic.

It's unknown, however, how powerful Sidhe magic is compared to the magic of the Old Religion, to the Dragonlords magic and to Troll magic. According to Gaius, Sidhe magic was ancient and powerful enough to possibly be able to negate the dragon magic of Mordred's sword: the physician told Merlin to take the dying Arthur to an isle in the midst of the Lake of Avalon, where he could have been healed. However, Emrys didn't make it on time and the king died (The Diamond of the Day: Part Two).

The known characteristics of Sidhe magic are:
 * The ability of cursing other Sidhe to live a mortal life in a mortal body. This happened to Aulfric and his daughter Sophia, because he had killed another Sidhe (The Gates of Avalon).
 * The fact that when they cast a spell or are particularly angered, their eyes turn red. Gaius noticed this when Aulfric caught him with his Sidhe staff. Sophia's eyes (and Arthur's) also turned red while she was chanting a love spell (The Gates of Avalon).
 * They can control humans through powerful love spells. Sophia used one to enslave Arthur's mind (The Gates of Avalon).
 * They can summon the Sidhe elders or open the gateway to Avalon through the Lake of Avalon. Grunhilda, a pixie, opened the lake-gateway with the incantation "A shruith inn Sidhe, is Grunilda messe! Dot-iccu la mòrumaldoiti do aiscid dàlae frit-su!" when she had to tell him what was happening in Camelot (The Changeling). Aulfric also used the spell "Dotiag-sa ar idbairt do denam" to summon the Sidhe elder and talk to him (The Gates of Avalon).
 * The ability of sacrificing a human to gain back their life of immortality. Aulfric cast this spell, while Sophia used her power on Arthur to make him drown in the Lake of Avalon: "Ia bend dǽdon níwe. Cúðon gare íewe deahl sǽ áre. Sé áre. Ig bæþ deahl sǽ néah. Déaþ ór cwylþ óga him. An wén. Flíete á. Dómdæg. Déaþ ór cwylþ óga him. An wén. Flíete á. Dómdæg. Déaþ ór cwylþ óga him, Arthur Pendragon. Ia bend dǽdon níwe. Cúðon gare íewe deahl sǽ néah. Sé áre. Ig bæþ deahl sǽ néah. An wén. Flíete á. Dómdæg. Déaþ ór cwylþ óga him". Aulfric said that once this ritual starts, it cannot be stopped, and if the victim's life isn't offered, the Sidhe elders would take the one of the performer (The Gates of Avalon).
 * They can use Sidhe staffs to cast spells and kill other Sidhe with their bolts of energy, but they can only incapacitate other magical creatures with them. This form of magical energy can be cast with a spell or non-verbally. Merlin used the incantation "Swilte, gold beorþ" and "Acwele" to do this, while Aulfric uses the incantation "Má réðe cine týne" and "Na þing biþ" (The Gates of Avalon, To Kill the King, The Changeling).
 * They can implant a faerie in little infants and thus create a changeling. The spell used by the Sidhe elder to enchant Princess Elena was: "Attrab i n-ingin-seo ocus oentaig lé" (The Changeling).
 * Their magic also allows them to cast various spells and enchantments (like locking/unlocking spells) (The Changeling).