Roger advanced on Samneric "...as one wielding a nameless authority". Why is it nameless?
If you are asking what the nameless authority is, in my opinion, I would assert that authority to be the Devil, evil, or the force that seems to be infecting all the boys as it radiates itself from the "Lord of the Flies". We know that this reference is also used for the Devil himself and I would assert that Golding uses the term "nameless" to describe the authority because Roger has no recognition of that which has taken over him in this last chapter. He has certainly been capable all of the sudden of greater purposeful harm than has yet happened so far in the novel.
Another perspective worth noting is the fact that when people use fear to intimidate or assert power over each other it is often the most terrible kind of authority that exercises no compassion or remorse for people who would be under someone.
Remaining a nameless authority keeps the ideas that the boys have no concept of the truths that they are destroying about humanity. It is always easier that way.