If this is in-game, Quick Claw.
If this is competitive, either Leftovers, Weakness Policy or Expert Belt would work better than either of those 2 items.
Also, I would suggest the moveset to be:
-King's Shield
-Shadow Sneak
-Sacred Sword
-Swords Dance
It doesn't get any STAB from Night Slash or Rock Slide, but gets STAB priority from Shadow Sneak and good coverage from Sacred Sword.