One execution of Trinity smash usually destroys the shield of a Death Knight and leaves them open to attack from any weapon. It does massive damage and gives a great boost to style rating. When you pause you will see a gleam going down the blade of Arbiter, this is when you press for the third time (still holding ) for the final attack of trinity smash. In my opinion the best ability for Arbiter is the Trinity Smash. While in the air they remain prone and cannot block and/or counter. Probably the best way to kill them without yet having a heavy weapon is to wait for them to hold their arm back before they're about to strike, hold to use Hightime, then Aerial Rave (, ,, ) on them in the air. You will face your first Death Knight (possibly more than one depending on difficulty) in this room before acquiring Arbiter.

A hallway on your right-hand side will lead to a room with Sparda's portrait in it. *** Spoiler - click to reveal *** When you enter the main hall in the house with the grand staircase, go up the stairs and to the left.