Stormraider's Surge

Dealing 25% of a champion's maximum health within 3s grants 48% Move Speed and 50% Slow Resistance for 4s. Move Speed is 75% effective for ranged champions. Cooldown: 20s - 10s

Tree

Sorcery

Row

Keystone

In short

Dealing 25% of a champion's maximum health grants a burst of MS and Slow Resist.

Patch

16.16.1