We first encounter this expression in Chapter Twelve, which is after Pip has had his fight with the pale blonde boy, whom he does not know the identity of as yet. He goes home and is very afraid, and when he has to go back to visit Miss Havisham, the fear reaches new heights:
When the day came round for my return to the scene of the deed of violence, my terrors reached their height. Whether the myrmidons of Justice, specially sent down from London, would be lying in ambush behind the gate?
Myrmidons is the name given to the followers of Achilles who fought under him during the Trojan war. However, it is also a phrase used to indicate a follower who obeys rules mindlessly and without thinking about the consequences of those rules. What is interesting about this allusion here is that it is linked to Pip's sense of guilt about having committed a crime, which is something that dominates the novel and foreshadows his involvement with Magwitch.