Deathfire. Deathfire Deathfire Deathfire. It's absolutely amazing on her.
Kat has quite low single target burst, but Deathfire gives you a gigantic power spike in that you can blast one guy into oblivion every 90 seconds. It also has a little CDR and 120 AP. The added single target damage of DFG allows you to start your kill chain much easier, giving you that first reset which leads to more.
Liandry's isn't bad if you do a tankier build (which can work) with Rylais, but it's not that good if you don't have Rylais. You have no movement impairing effects, and with the build above, you won't be tough enough to take advantage of the added damage. Liandry's is an item for if you're going to deal your damage slowly, not for trying to burst like a mofo.
Void, Zhonyas and DCap are all good. The order you get them in can be very important (I often get DCap last), but building like that makes you very frail, which the Abyssal scepter won't really save you from. The massive stack of MPen you have here is nice, but you could get a DFG in place of Liandry's and be much more effective in the mid game. Especially if DFG is your rush item. Finally, if you get one major item to help you live (
is most popular because you can get resets while reviving, though other options work too such as
or
) you can be really terrifying while having numerous ways to stay elusive and, more importantly, alive.