As per Tibia Fandom, she is described as:
"Daughter of the mighty sorcerer Ferumbras and granddaughter of Fermuba. Not much is known about this woman.
Her kidnapping by Nightmare Knights is possibly what triggered Ferumbras' first attack on Thais. Fortinbrae was then being hidden in Carlin. During Ferumbras' second attack, they had to kill her and retrieve something from her corpse before they were able to finally defeat him.
Occasionally, during Test Servers, Cipsoft member used to log onto character called Mistress Fortinbrae to attack other players in Thais and Carlin, similarly to how Ferumbras did it in the old days of Tibia. The character was a Sorcerer and had special abilities like unusually high base speed, very strong self-healing and ability to summon Demons, Hellhounds, Hellspawns, Nightmares, Zombies and Essence of Darkness. "
However looking at the the story told by Knightmare (here), it might not be true since he says:
"We did have his "daughter" Fortinbrae appear for one or two occasions, but somehow she never even remotely acquired the fame that her father had accumulated."
and uses word -daughter- in quotation marks, which leaves a room for interpretation that this is just assumption by others, possibly not true.
In addition, the simple fact she appears a a Mistress... Is it possible that she is Ferumbras mistress?
Do you have any information's regarding Fortinbrea?