Thais was first mentioned in the novel
Worlds Without End, and appears to be the offspring of Aina, an immortal elf, and Ysrthgrathe, a powerful Named Horror. Apparently his mother abandoned him at some point. By his own account, he was behind the first Great Ghost Dance in the nineteenth century. He was next spotted in
Loose Alliances, where he was operating in southeast Asia (he's said to have good relations with the nagas of Angkor Wat) and to have clashed with the Great Dragon Arleesh.
As a hybrid, Thais shares aspects of both of his parents: apparent immortality, considerable magical powers, and it has been shown that he can be summoned. He may or may not bear similar qualities to the Ulkmen of Earthdawn, who are also said to be descended from Horrors.
Notably, Thais did
not make an appearence in the novel
Scars.
The legend of Thayla-who has never been shown to be elven, by-the-by-was first related in
Harlequin's Back and provided part of the background for that adventure. Thayla was apparently killed in
The Dragonheart Trilogy, each book of which contains a copy of the legend of Thayla that was unearthed and translated by the Dunkelzahn Institute of Magical Research.