What are myrmidons of Justice" in Great Expectations?"

Myrmidons were Thessalian warriors who fought under their king, Achilles, in the Trojan War. Modern usage of myrmidon (note the lowercase m) applies to an unquestioning follower or subordinate. In Great Expectations, when Pip mentions myrmidons he simply means policemen. He's afraid he'll be arrested for fighting with the pale young gentleman at Miss Havisham's:
When the day came round for my return to the scene of the deed of violence, my terrors reached their height. Whether myrmidons of Justice, specially sent down from London, would be lying in ambush behind the gate? Whether Miss Havisham . . . might rise in those grave-clothes of hers, draw a pistol, and shoot me dead?