Flickerstep is a Unique Boots in Diablo IV Season 15 for All classes. It comes from General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet).
Unique power
Each enemy you Evade through reduces your active Ultimate Cooldown by [3.0 - 4.0] seconds, up to 10 seconds.
Affixes
Primary Core Stat
Attacks Reduce Evade's Cooldown
Where it drops
General Unique Pool - every Lair Boss (Initiate, Greater, Exalted, Belial) can drop it in addition to their dedicated table (one guide's Boss Loot Table Cheat Sheet)