Lethal Tempo Is Likely Unbalanceable

If it's viable, it becomes OP on rangeds like Twitch, Kog. It's too synergistic with specific champions and especially ranged ones. This will force it to be balanced around those OP cases (unless the champions are nerfed for it and then they HAVE to use Lethal Tempo to be viable). This would make it weak on everyone else and especially melees who may get kited during its duration and in general cannot use it as safely (thus probably not benefiting from the full 6 seconds as much as an Ashe etc. since they'd just die).