The Euchdag (also known as the Diamair) is a creature that carries a great burden – the gift of all-knowledge. She is the last of her kind; it is for that reason that the Euchdag went into hiding.
Note: Although not revealed the Diamair was actually the young White Dragon Athusia in a form in which she could speak and heal others, yet unable to heal herself. The white dragons being "Rare indeed", only White Dragons could change their form. Athusia being the last of her kind. When she first met Morgana she was very young and did not realize at first the hatred in Morgana's heart. She was captured along with Morgana and held prisoner with Morgana for two years in a pit. While she grew, the prison was too small and caused her to be crippled and twisted. Once she realized Morgana's true plan, she could never let Morgana know that she was the Diamair that Morgana sought and had to "hide" from Morgana in plain sight. She only revealed herself in her other form to Gwaine and Merlin telling Merlin that Arthur himself was his own bane in life.
Morgana was looking for the Diamair, the key to all knowledge, so that she could find out what Arthur's Bane was. With this information, she would have been able to find out how or who by, Arthur will die. Morgana didn't know that the Diamair was in fact the Euchdag, yet ordered the prisoners the Saxons captured, to dig in the mountain to find it. Gwaine saw a glowing in a cave and went to inspect. It was there he found the Euchdag, but was brutally beaten by the Saxons. As Gwaine was injured, the Euchdag found him and cured him of his wounds. When Gwaine woke up, she explained to him that her kind was shunned and hunted for hundreds of years and as a result she is the last one alive. When Arthur and Merlin arrived to rescue Gwaine, the Euchdag fled.
Euchdag later appeared to Merlin, referring to him as Emrys and assuring him that all that has happened has been for something few have ever wanted, something Morgana never found and never will find. When Merlin realizes she is the Diamair, the young warlock acknowledges that his burden is nothing compared to hers, even Euchdag herself saying that her destiny is both a gift and a burden. Asking the young warlock if there was anything he wanted to know, Merlin didn't ask anything, believing it would not be good. Euchdag respected Merlin's wisdom, saying that his wisdom will last a long time in the minds of mankind. However, Merlin does ask her one thing: who is Arthur's bane if Mordred isn't. Euchdag answers that is it Arthur Pendragon himself (Arthur's Bane).
- She has been also known as an 'alien' by fans.
- The Euchdag's gender is not specified, but fans refer to it as female because her anatomy resembles that of a female human. There is no guarantee that her kind have the same anatomy, including gender organs, as humans. Her kind may not even have specified gender.
- According to MacBain's Dictiony (Irish), Section 16, "a fair maid, a charmer: "featsome one", from euchd".