Sidhe

The Sidhe (pronounced "shee") are an immortal race of Avalon, the Island of Youth and they appear as small blue humanoid winged creatures. Gaius describes them as "masters of Enchantment and a cruel race of beings." They were served by Pixies and could live for 1000 years.

A Sidhe elder started a 20 year plot using the daughter of Lord Godwyn, Princess Elena. A short time after she was born a Sidhe elder came and cast a spell on her. He then had his Pixie servant, Grunhilda disguise herself as a Human and become her nurse. Grunhilda regularly sprinkled her with dust as she slept so that when the time was right, the Sidhe inside her would completely possess her. The right time being when she married Arthur Pendragon. (The Changeling)

They banished Aulfric and Sophia from Avalon giving them human bodies and a mortal existance. Alfric wished to allow Sophia access to return to Avalon and convinced her to enchant Arthur Pendragon so that they might sacrifice him to the sidhe, who required the soul of a mortal prince. Merlin turned up at the last minute and saved Arthur by attacking and killing both Aulfric and Sophia with Sophia's sidhe staff. (The Gates of Avalon)

''As these Sidhe had no problem with Aulfric sacrificing Arthur, who as explained was vital to another Sidhe's plan. Not all Sidhe are likely afiliated with oneanother.''

When Elena was 20, Godwyn and his old friend Uther Pendragon organised an arranged marriage between Arthur and Elena. Merlin soon discovered the truth, when Grunhilda informed the Sidhe elder of this he went to kill Merlin in his sleep but Merlin destroyed him using the sidhe staff that previously belonged to Sophia. Gaius then created a tonic to get the Sidhe out of Elena, Merlin then killed Grunhilda and the Sidhe. (The Changeling)

Historicity
In Celtic (especially Irish) mythology and folklore the Sídhe are also known as Aos Sí, and sometimes as the Túatha De Danann. They are similar to the Elves of Norse mythology (as, to some extent, portrayed in Tolkien's works on Middle Earth), and are not at all like the small pixie-like creatures depicted in Merlin. They inhabit the Otherworld (which in Welsh mythology is known as "Annwn" and may well be linked to "Avalon") and are immortal, but not necessarily evil. However, they do tend only to care about themselves even when dealing with Men, and iron and steel are traditionally inimical to them.