Esteril Thyssen was the king of Kovir and Poviss, being the first of the Thyssens to sit on the throne, having ascended to it after the death of King Gerard Troyden of Kovir, the last of the House of Troyden. He was the son of Edmund Thyssen and Alicja Troyden and nephew to Ramona and Berengaria.
He married Princess Agnes, daughter of King Videmont of Aedirn. Their relationship was a truly romantic tale. When the young couple first met (on the road leading into Ard Carraigh), Esteril was "only" a Count, the son of a "mere" Margrave and was therefore considered entirely unsuitable to court the princess. The fact that Esteril's mother came from royal blood did nothing to sway the king's favor. Videmont was very conscious of the fact that the man who married his daughter would inherit the crown, so not just any man would do. When Esteril tried to formally propose marriage through his father, the proposal was refused. The proud Kovirian margrave had no intention of exposing himself or his son to the shame of such a refusal, so Esteril was forbidden to even so much as look at the princess again.
These measures didn't work as the young couple were attracted to one another and refused to comply and were both careful enough to hide their relationship and not give anything away in front of the kings. Esteril, posing as a friend of the king's sons, was actually well received by the king in Vengerberg. Videmont was unaware that his children were in on the deception.
One night, Agnes and Esteril simply fled together on horseback. Legend says that to avoid the wrath of both their fathers the two hid themselves at Kaer Morhen, the witcher stronghold, but this is unlikely. It is, however, a historical fact that a year after the lovers eloped, Benda, the king of Kaedwen, informed both Videmont and Edmund that the two were now grandparents. The grandson was of Thyssen blood and named Baldwin in honor of his great-grandfather, the famous king of Aedirn.
Videmont felt slighted and sulked for some time. After the death of King Gerard, Esteril was hailed as king of both Kovir and Poviss, finally elevating him to a "more suitable rank", even though the union was a fait accompli. Videmont did dote mightily on his grandson and later, great-grandson who were born as a result of that union.