QQ about darius buff already
let's talk about the buff and why he needs it for a second.
Noxian Guillotine (R) [Still looking, might be more] Mana cost reduced to 100/50/0 New Effect: At rank 3, Noxian Guillotine's cooldown resets entirely on kill
there is no damage buff. so his level 6 stays the same, and still has a 12 second window to cast his ult again on kill. at level 11, the mana reduction really helps out, as darius does not stack mana, and a 100 mana ult really takes a huge toll on darius's mana pool(not to mention his eqw cost a decent amount of it, and tend to drain his mana while harassing enemies anyway). still a 12 second reset.
at level 16, the mana cost is 0, and gives a full reset on KILL with it. Darius desperately needs this. how many times late game in the league of mobility does darius end up getting a kill with his ult, and then league of mobility comes into play and everyone just avoids him for 12 seconds until his damage is negligible? ill tell you. almost all of the time. with the reset on his ult at max rank and no mana cost(not 100% sure on have 0 mana cost), this keeps darius relevant late game as there is the threat of him ulting you. this still doesn't fix the problem of say, i don't know ALMOST EVERY META CHAMPION RIGHT NOW BEING 2-3 TIMES MORE MOBILE THAN HIM.

(not insanely more mobile, but twice the range, an escape in her e, and a root in her w)
the only thing this buff does is keep darius relevant in teamfights late game where he is laughably bad. you wanna cry about manaless resets on a nuke ult? o hai
, the only difference is darius still requires stacks to do damage, which takes time. meanwhile kat can just wait until you blow your cc on someone else then q+e+w+r, get one or 2 kills, rinse and repeat.