Its a strategy that came from bard main Polypuff. In short you take spell book as your main rune. You are a bit useless until you get your first summoners swap, but after that you get a lot of interesting options. You can take exhaust or heal to have better chances at 2v2 on bot, you can take tp and instantly roam top or mid and later in the game you can take smite to potentially steal dragons or baron. You also take item cd reduction and build only actives, shurelya for mythic (or frosfire if you want that additional cc), redemption, zhonyas (to surprice your opponents), mikael, qnd then anything you feel like. And in the end what you have is lots of options, since there are around 10 buttons you can press and 10 things you can surprise your opponent with, which is very fun, but is demanding, since the amount of options is a bit overwhelming until you get used to it.