Yen's Blessing is a Unique Boots in Diablo IV Season 15 for All classes. It drops from Urivar.
Unique power
Casting a Skill has a [40 - 60]% chance to trigger a Non-Mobility, Non-Ultimate Skill that is currently on Cooldown. Can only occur once every 8 seconds.
Affixes
Primary Core Stat
Movement Speed
Where it drops
Urivar (Exalted Boss, 1x Lair Key) - dedicated multi-class Unique
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)