Fans of The Shadowhunter Chronicles may already be familiar with The Seelie Court. That goes double for those who have been following the TV series Shadowhunters: The Mortal Instruments.
The Seelie queen is the sometime ally, sometimes antagonist of the protagonist teens. Operating with her own agenda, the Seelie queen appears with different faces at different times.
However, the writers only borrowed half the folklore. The Seelie Court comprises of the light fay. They only take umbrage with humans who slight them. Other than that they tend to look out for humans even helping them in their own way.
The real terror are the dark fay of The Unseelie Court. Their attacks on humans need no provocation. They will play sometimes deadly or damaging tricks simply for their own amusement. Though they sometimes grow fond of individual humans – treating them as pets – they are largely to be avoided.
Neither of the Seelie courts have made a direct appearance in Legend. (Yet).
To find out more about the Seelie Courts (both of them), you might like to start with Wikipedia’s classification of fairies.
