CDR Quints

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struggles violently muffled from wrestling, you hear: "mmf...but if it was a tank build-"

There's the problem xD Renekton is a caster, yes he should use CDR considering that he doesn't have mana costs too. The only thing is, If you run full CDR runes, His AD will be low, which means his healing from his Q will be too, His damage really low, and his sustain will drop HEAVILY, And Renekton is meant to be a bully in lane, punish the enemy and push them out of lane, You can't do that with a full tank build in my opinion.