"Five kings are coming to this city."

Translation: Tōmyz dāryssy keso oktiot umāzisi.

[AulusBurns]

Kesa: adj. I, singular terrestrial locative: kesot (source: wiki.dothraki.org). So I wonder if "kesot oktiot" is correct, and not "keso oktiot" as it is now.

[KyleSmith-Laird]

It says on wiki.dothraki.org as a footnote (#8) after singular terrestrial locative for bona (which applies equally to kesa): "The -t is omitted before a consonant, but retained before a vowel," so yes, according to the wiki, it should be omitted.
Source: http://wiki.dothraki.org/High_Valyrian_Adjectives#cite_note-t-7
In addition, on the High Valyrian here on Duolingo, it concurs: "Again, for singular forms that end in t (excluding those that end in ȳrot) the t reappears when the following word begins with a vowel."
Source: https://www.duolingo.com/skill/hv/Locative/tips-and-notes
Good catch!

[ReinerSelb]

i think it should be "tōmys" because of the following "d".

[purplepurpur]

No, the Z only transforms into an S after a voiceless obstruent (s, k, p, h, th, kh, probably among others). The D is a voiced obstruent (d, g, b, gh, probably among others) and thus the Z should be retained.