Unless you want to go AP aftershock provides better benefits. Early on it makes you more tanky helping you survive tougher lanes/ all ins.
Late game the extra aftershock tank stats should allow you to chase people with your ult, ensuring they don't walk out if you Zhonya's asap.
Moonstone is really easy to abuse with your W. I recommend going for it's health components first as that extra 20ap won't be doing that much damage. An early fairy charm helps with your W spam

Moonstone works best when versing or playing with tanks. Shurelyas is if you're the only engage on the team. I'd invest in a second item Zhonya's then. Locket is the way to go if you're against assasins you deem a threat. Locket a Katarina's ult and you're good luv. If they're not fed don't bother.
Make sure you are building healing reduction if your team needs it. You proc it very well with W and Demonic burn damage. Since you're not building damage its better no one else cripples their's.