nimbus cloak will be so good on mage and lot of champion .

For example lighting rush give him 100 % of his total movement speed . so instead of getting 100 ms you get 200 ms in lighting rush form and ult also combine with celerity its gunna be some good ms bonus damage .

Twisted fate would normally use flash to get close and stun his opponent when using ult to gank so it might close the gap .

aurelion sol getting more free ms to follow the enemy or even flee away from the danger when ulting .

his passive and this rune gunna maybe help fiddlestick to close the gap without using flash when ulting .

the slow on her ult + the MS might make the ability even more succesfull .

I just threw some mage but the keystone itself might be usefull to larget poll of champion .

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Movement speed is a valuable stat and can just give you the niche you need to deal even more damage by getting maybe 1 or 2 more auto or getting in range for a taunt , getting faster to your target getting another stack of movement speed or sticking to your target .