Viper + Io

Why this works

Giving Viper more attack speed with Overcharge isn't the main benefit of this pairing. Viper's damage comes from his corrosive spells and Nethertoxin's passive break, neither of which scale dramatically with attack speed. The real value Io provides is mobility and map presence. Tether gives the otherwise slow Viper the speed needed to position himself for a perfect Nethertoxin or to stick on a target. More importantly, Relocate turns Viper into a global threat, letting him bring his heavy single-target lockdown from Viper Strike to any fight, overcoming his inherent lack of mobility.

The duo relies on snowballing from an early advantage. With Io's sustain, Viper can dominate his lane and then use Relocate to secure kills before the enemy can react. The problem is that the combination doesn't scale as well as others into the late game. Opponents will quickly learn to burst down Io, leaving Viper stranded and easily kited once the Tether is broken. In full-scale teamfights, Viper's single-target damage can feel insufficient, and Io will often have better allies to empower. The pairing's success depends entirely on whether they can build a lead before the enemy's teamfight composition comes online.