You can RP any non-key NPC characters you want, regardless of their realm affiliation. You can RP that a cult of tree-worshiping savages in the Perdan Mines were butchered by local officials who thought they were Sirionite sympathizers. You can RP a group of commoners in Greatbridge who slaughter a traveling circus of Sirionite jugglers. You can RP the actions of the prison guards while you are in the enemy prison.
What you shouldn't do is RP other character's actions, other character's soldiers' actions, highly-placed officials (i.e. the king's manservant, or the judge's head scribe, etc.), or any NPC that someone else created. (Like this Fandor guy that Atanamir has already created. You can't RP his actions without clearing it with Atanamir beforehand, because Atanamir created the character.)
