ταῦτ' εἶπε και αὐτίκα ἄγγελον ἔπεμπεν ἐπὶ βουκόλων τῶν Ἀστυάγους ὃν ἠπίστατο νομάς τ' ἐπιτηδειοτάτᾱς νέμοντα καὶ ὄρη θηριωδέστατα.

[jmorsay]

ταῦτ' εἶπε και αὐτίκα ἄγγελον ἔπεμπεν ἐπὶ βουκόλων τῶν Ἀστυαγους ὅν ἠπίστατο νομάς τ' ἐπιτηδειοτάτᾱς νέμοντα καὶ ὄρη θηριωδέστατα.

My translation: If even this man really was sending a messenger to the herds men of Astuages whom is understood to be the most servable to laws and most occupying with laws and most infested with wild beasts.
          
Questions
1. I don't understand the part"ὅν ἠπίστατο νομάς". What is the subject of the relative clause? I don't see a nominative in the clause.
2. What does "θηριωδέστατα" mean?
3. Is this correct?

[billberg23]

1.  εἶπε is 3rd pers. singular aorist active indicative of λέγω.  So ταῦτ' εἶπε = "he said those things."
2.  ἐπὶ βουκόλων τῶν Ἀστυάγους ὃν ἠπίστατο ... = ἐπί τινα τῶν Ἀστυάγους βουκόλων, ὃν ἠπίστατο ... = "to (one) of Astyages' herdsmen whom he knew ..."
3.  ἠπίστατο is 3rd pers. singular imperfect middle indicative of ἐπίσταμαι, "know."  Sorry it's not listed in the vocabulary in the back of Allen's book;  didn't mean to throw you a curve ball there!
4.  Distinguish νομή, "pasture," from νόμος, "law."
5.  The participle νέμοντα follows ἠπίστατο:  "knew (to be) dwelling in."     
6.  ἐπιτηδειοτάτᾱς is fem. plural superlative of the adjective ἐπιτήδειος, "suitable."  The pastures were "most suitable" to Harpagus' plan.
7.  θηριωδέστατα is neuter plural superlative of the adjective θηριώδης, "infested with wild beasts."
8.  ὄρη is neuter plural of the noun ὄρος, "mountain."

Want to try it again?

[jmorsay]

My translation: He said those thing and however and however he sent to a herdsman of Astuages whom he knew to be dwelling in pastures and most suitable and most infested in wild beasts in the mountains.

Is this correct?

Thank you

[billberg23]

My translation: He said those things and however and however immediately he sent a messenger to one of the herdsmen a herdsman of Astuages whom he knew to be dwelling in pastures and most suitable and in mountains most infested in wild beasts in the mountains.

Your second attempt was a lot closer, JM!
How did you get "however" for αὐτίκα?
You have to pay really close attention to endings, JM:   ἐπιτηδειοτάτᾱς goes with νομάς because both are feminine plural accusative;  θηριωδέστατα goes with ὄρη because both are neuter plural accusative.  Both νομάς and ὄρη are direct objects of the participle νέμοντα. 
To review the declension of neuter nouns like ὄρος, go to p. 239 para. 35.  To review comparison of adjectives, go to p. 244.