So what Rook is doing is sending a concentrated blast of Dark Side emotions and their related Dark Side effects, mixed with his own experiences of murder and Darkness. This is something he's perfectly capable of doing, as he's extremely practiced in Telepathy and has transmitted emotions and memory fragments before. If Sio has had formal training against mental attacks, she might be momentarily disoriented. If not, she would likely be sent mentally reeling, and maybe down the path of the Dark Side once again if that's a road you'd like to explore. This is an opportunity for you to get creative with what she experiences mentally against his onslaught.