In this new season, many items were completely changed or even removed. Including our precious Gunblade :(. In order to compensate for this, I have found that the new Tiamat is very useful for Katarina. Since they buffed her and made E and R now do on-hit, Tiamat can proc very easily and I find it very useful on Kat. The duskblade is definitely questionable, but I think that it can work when you really want to just destroy the enemy team because if you get low, you can go invis and proceed to one-shot anyone how could be a threat to you if you have enough damage.