"The Targaryens are strong."

Translation: Targārī kostōba issi.

[SterE666]

Why not kostōbi?

[xPsGoDWh]

Because "Targārien" is a terrestrial noun. And for terrestrial nouns, the plural from of Class I adjectives in the nominative is "kostōba". It's not that obvious for this nouns, but the plural form of "naejon" is "naeja", so it agrees with that, sort of.