ὃ δέ μοι δοκεῖ καὶ ἐκείνων κάλλιστον εἶναι, τούτου μνησθήσομαι, ὅτι τὸ ἑξῆς ἔργον τοῖς Μαραθῶνι διεπράξαντο.
I shall mention what out of these seems to me the most beautiful, the deed that they accomplished just after Marathon. Incorporation of ἔργον into its relative?