Is this a Mecha Fiddlesticks or Karthus?

Mecha Kha'zix.

You can actually see Mecha Kha'Zix's silhouette behind the blade. The three small images in the post are the Mecha skins that already exist (Aatrox, Malphite, and Kha' Zix).

Aside from what was said already,there is also the fact that Karthus doesn't have a scythe(only in one of his skins,but the same goes for all reaper skins)

The new mecha is rengar

Mecha Death Syche from GW

Mecha Fiddlesticks ROFL

RIOT PLZ

Oh boy

Mecha Thresh seems to be the best bet in my opinion.

Plz, clearly Mecha