Who wins Vel'Koz vs Karthus?
MID.GG has 5 high-elo VODs of this matchup so far, not enough games yet to call it.
Across 31 NA and EUW ranked games in bot lane, Vel'Koz wins 58.1% against Karthus. That is too few games to compare with the 54.5% Vel'Koz wins in that role overall.
In our library of 5 high-elo VODs (latest patch 26.21), Vel'Koz won 40% against Karthus and averaged 10.6/5.4/10.4. That is not enough games yet to say who the matchup favours.
The most played Vel'Koz setup into Karthus takes Arcane Comet (Sorcery and Precision) and builds Blackfire Torch, Shadowflame and Rabadon's Deathcap. It shows up in 4 of 5 games with a 50% win rate. The main alternative is Dark Harvest (1 game).
Taken from the 5 VODs on this page, patch 26.21.
Every streamer in the library with ranked Vel'Koz games, most games first. Each links to their VODs on MID.GG.
MID.GG has 5 high-elo VODs of this matchup so far, not enough games yet to call it.
The most-played setup is the Arcane Comet rune page, taken in 4 of the 5 high-elo VODs on MID.GG, with Blackfire Torch, Shadowflame and Rabadon's Deathcap as its most common core items.
The most recent games in this matchup, newest first.

