New item: The Showstopper - to counter League of Mobility!

Stats: 100 AP, 10 % CDR

UNIQUE active: Deathgrasp: "Fires a bolt in a target direction [SKILLSHOT]. For every 100 units you have traveled less than the first champion struck the last 3 seconds, deal 3% increased damage to the target while receiving 3% less damage from the target. If the difference is greater than 500 units, additionally root the target for 1 second. 80 second cooldown

Price: 3400 gold

This is a way for immobile mages to counter all the highly mobile assassins on the rift, by giving the opportunity to kill them IF YOU HIT YOUR SKILLSHOTS, making both the assassin and the mage able to kill each other during a duel - for example, if you are vs she can jump in, use her combo and get out even if she gets stunned and her charm juked. This would ensure if Ahri doesn't outplay the Xerath by juking his skillshots, she would die - and if she did, Xerath would die as normal. The active doesn't need to be on an item, but could also just be on a champion (Xerath would actually be a great idea, on his E)

The active shouldn't count in ults from TF, Panth, Tahm and Reksai and TP to avoid some really extreme interactions. The % increase/decrease amount is up for discussion, 3 % is maybe a little much, and i am not sure of the root either.

What do you think? Should we have more counters for mobility in league? It would for sure create more tense duels between mobile and immobile champions, where mobile champions HAS to juke or get killed. This is like a conditional DFG, but we just had one of those added into the game - could this item/effect be useful as well?