A suggestion for a Karthus change

I feel like should be rewarded for landing consecutive Lay Wastes on a champion so here is something I came up for him, which took inspiration from the Vladimir Q rework

>After successfully landing 3 Lay Waste on a champion, the next Lay Waste within 5 seconds grants Death's Grasp

Death's Grasp : Lay Waste roots champions for 1/1.1/1.2/1.3/1.4 seconds and Karthus is briefly hasted for 1 second, gaining 50% bonus movement speed which quickly decays.

The root cannot affect the same target for 5 seconds and any other hastes gained during that duration are reduced to 25%

This solves the problem of Karthus' targets always avoiding him do to the huge mobility provided these daysn and also finally gives him a hard CC, something invaluable to mages