"Bisys azantys ēdrus."

Translation: This knight is asleep.

[AdamScott794079]

Why the "S"?

[SprightBark]

You mean on the end of bisys, right?

It's an adjective here, because it modifies a noun, rather than standing as a noun in its own right.

Adjectives agree in gender, case and number with the noun they modify.

[The quote is from the top of the page linked to below.]

Solar nouns (which themselves often end in -s in the nominative singular) often cause -ys to appear on the end of class 1 adjectives (of which bisa is a subclass) for the nominative case (and the vocative).

I think bisa follows the class 1 adjective pattern when it is before the noun: https://wiki.dothraki.org/High_Valyrian_Adjectives

And its own subclass pattern when it is after the noun: https://wiki.dothraki.org/High_Valyrian_Adjectives#Bisa-type

But for further confirmation see also: https://wiki.dothraki.org/High_Valyrian_Pronouns#Demonstrative_and_interrogative_pronouns