"I hear the knight."
It is indeed Azantys in the nominative case.
High Valyrian has a case system. The nouns (and some other words) change to show which case they are in.
Azanti is the accusative singular, as seen in this sentence.
It follows the same pattern as other verbs which end in -ys in the nominative singular: https://wiki.dothraki.org/High_Valyrian_Noun_Declensions#Solar-type:_loktys