Either Darius needs buffed or Pyke needs nerfed, currently theres no reason to play the former.
Darius is a juggernaut who is a tanky target because of his ult decimating fights. Pyke is a support who is slippery, and does the same thing but better. His ult is AOE, can splash, is a dash, and gives double kill gold. All of these things better than Darius.
Right off the bat, Pyke ult almost completely matches Darius's ult in base damage, but better because it scales with level rather than rank. Pyke also double dips off all his items for his damage scaling on it.
With
level 1 ult is 260 damage [200 + (40 x 1.5)].
With
level 1 ult is 306 damage [250 + (50 x .8) + (8.8 x 1.5) rounded] . Already his Ult is a sizeable chunk better than Darius. This gap is further spread until level 11 when Darius gets his level 2 ult, which would be:
With
(If you went an extremely aggressive build) is 580 damage [400 + (120 x 1.5)].
with
is 559 damage [430 + (110 x .8) + (27 x 1.5) rounded]. With an extremely aggressive Darius build, he just barely beats Pyke's ult. This again, is further spread until level 16.
At which point, Darius needs about 3 offense items against Pykes 4 to compete with his ult damage. This is WITH pykes benefit of AOE, resettable, dashing, gold giving execute. Darius will not be building that much damage, so almost all the time pyke will be doing more.
There clearly needs to be a power budget here because both compete for the same role but one does it by far better almost all of the time