This is what’s known, sometimes, as a yena-tena construction. The destination is correctly placed in the nominative.
The ‘yena’ has the sense of ‘where’ or ‘by that way’, and the ‘tena’ is ‘there’.
So in this sentence khattiyā (nom plu) is really the subject of its own clause.
Literally something like, where those khattias were, there he approached.