Empty slot is for you to decide really, but I kept a dash polarity because it lets me swap between [Augur Message], [Augur Secrets], [Stretch], or [Streamline].

Since his [Flechette] orb damage scales with enemy level, crazy high strength isn't needed, I included [Umbral Intensify] to offset [Overextended] and to boost the other Umbral mods, but if you want to rock [Augur Secrets] for the memes and the extra amount of people Bastille can hold up you can with the flex slot.