Once Void Rift is in hand, stack magic pen — damage explodes. Pop E snowball into the pile; main damage comes from Void Rift, so magic pen is priority. Liandry's can be skipped for Rabadon's.
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About Void Rift
Void Rift
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All Karthus combos
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More Void Rift Combos
Neeko S E, Q, and R all deal AoE damage on short cooldowns — perfect for Void Rift. 
Katarina S Katarina's R throws many daggers across three enemies in a short window — every dagger procs Void Rift, and one ult can trigger 70+ Void Rifts. Insane. The ult also triggers Void Rift's 99% slow, locking enemies in for the full damage. 
Udyr S Udyr's awakened R is a sustained AoE — every tick procs Void Rift, melting enemies in under a second. Bring Snowball, engage with it, then drop awakened R in the pile — Void Rift's slow + burst makes for huge damage. 
Heimerdinger S Turret attacks now proc Void Rift this patch — feel free to take it. 
Viktor S E and R are both AoE, perfect for spamming Void Rift procs. 
Hwei S Hwei's QE (magma) and ult are sustained AoE DoTs — every tick triggers Void Rift for instant AoE nukes. Pair with EE's forced reposition and Void Rift's 99% slow — enemies can't escape. 
Maokai S Void Rift is magic damage, so it fits AP champions with high magic pen — % magic pen amplifies it, and Pridestalker's ability damage procs it many times. Forge: magic pen > AP > MS. 
Ekko S Void Rift deals magic damage, so it fits AP champs with high magic pen — % magic pen amplifies, and Pridestalker's ability damage procs it many times. Forge: magic pen > AP > MS. Keep exploring
Karthus + Void Rift ARAM: Mayhem Combo Guide
This S-tier combo pairs Karthus with Void Rift in ARAM: Mayhem. This is one of the strongest combos available — pick it whenever you can.
Once Void Rift is in hand, stack magic pen — damage explodes. Pop E snowball into the pile; main damage comes from Void Rift, so magic pen is priority. Liandry's can be skipped for Rabadon's.