"The Unsullied's long sword is good."
Translation:Dovaogēdo korze sȳz issa.
From the lesson notes: "The genitive case is assigned to the possessor in a possessive phrase." There is no separate genitive/possessor in the sentence; they are one and the same.
Korze is the subject of the sentence; it is not describing (possessing) anything else, so there is no need for it to change case. Dovaogēdo is in the genitive because the possessor is the Unsullied, and what the Unsullied possesses is the sword.